Promising immunomodulators for management of substance and alcohol use disorders

2.1. Substance and alcohol use on the immune system and neuroinflammation

Over the last decade, there has been an increasing amount of evidence demonstrating the effects of substance and alcohol use on immune and neuroinflammatory responses [12, 15, 16]. While the molecular effects of each particular substance varies slightly, resounding evidence demonstrates that the use of methamphetamine, opioids, cocaine and alcohol can elicit the release of pro-inflammatory cytokines and induce inflammation in both the periphery and central nervous system [12, 16]. Many inflammatory markers overlap between the different substances and thus, immunomodulators targeting these substrates can be beneficial for individuals with SUDs that span across different classifications [12]. Below we discuss the overarching effects of methamphetamine, opioids, cocaine and alcohol on inflammatory markers and the known effects each has on neuroinflammation. Later in the review, we highlight important immunomodulators identified for potential treatment for SUDs and AUDs, which rely on their modulation of inflammatory markers discussed here. While beyond the scope of the current review, others have explored in detail various drug-immune interactions, and we direct readers to two recent reviews on this topic by Jones [17] as well as Bachtell and colleagues [18].

2.2. Overview of current treatments for substance abuse

There are no pharmacological treatments specifically approved for SUDs involving cocaine or methamphetamine; however, various existing medications approved for other purposes have been investigated but without overwhelming success. Such classes of medications that have undergone clinical testing include prescription stimulants such as modafinil, bupropion and methylphenidate, anticonvulsants such as topiramate and gabapentin, antioxidants and glutamatergic modulators such as N-acetylcysteine, and the copper chelator and acetaldehyde dehydrogenase (ALDH) inhibitor disulfiram [39–42].

For the treatment of SUDs involving opioids, approved medications are primarily opioid agonists used for medication assisted maintenance therapy. These medications exert full or partial agonist activity at MOR and/or DOR or KOR, and the most commonly used are methadone and buprenorphine/Suboxone [43], although other MOR agonists such as tramadol and the broad spectrum opioid antagonist naltrexone have also been used [44]. The mixed MOR inverse agonist and weak partial KOR agonist nalmefene was approved for opioid overdose treatment in 1995, but discontinued by the manufacturer in 2008, only to be reintroduced again in a nasal spray formulation as an opioid overdose countermeasure [45–47]. The long-acting MOR agonist and methadone analogue levo-alpha-acetyl-methadol (LAAM) was also previously approved by the FDA for opioid addiction, but some markets discontinued its availability starting in 2001 due to the possible emergence of ventricular arrhythmias [46]. As an aside, noradrenergic receptor modulators such as clonidine and lofexidine are often prescribed to individuals undergoing opioid detoxification due to their ability to mitigate severe sympathetic nervous system overactivation that accompanies opioid withdrawal [48].

The first medication to treat AUDs approved by the FDA was disulfiram, an ALDH inhibitor that prevents the transformation of the ethanol metabolite acetaldehyde to acetate, resulting in systemic accumulation of acetaldehyde during alcohol consumption which produces unpleasant effects such as dizziness, nausea, and vomiting [49]. Intended as an alcohol deterrent, there is currently limited clinical use of disulfiram. In 1994, the FDA approved the broad spectrum opioid receptor antagonist naltrexone for AUDs, with its moderate efficacy implicating the endogenous opioid system in mediating alcohol craving or subjective effects [50]. Naltrexone was eventually developed for intramuscular injection to increase medication compliance [51], and recently other opioid modulators such as nalmefene have also been investigated [52]. Finally, the taurine derivative acamprosate (calcium acetylhomotaurinate) received FDA approval in 2004 for AUDs, though its precise mechanisms of action remain elusive and its efficacy is moderate at best [53].

3.1. Major depressive disorder

AUDs and SUDs are more strongly associated with a comorbid diagnosis of major depressive disorder (MDD) than any other neuropsychiatric disorder [54]. Additionally, individuals experiencing comorbidity of MDD and SUDs display more depressive symptoms, poorer rates of recover, higher functional impairment and higher rates of hospitalizations than those with MDD alone [57, 58]. MDD has been associated with elevated pro-inflammatory cytokines, chemokines, as well as peripheral and central immune cell recruitment. Several preclinical models of MDD have investigated the efficacy of immunomodulators on depressive-like symptoms in animals. Minocycline, an α2 agonist, attenuated depressive-like symptoms as evidenced by decreased immobility in the forced swim test and decreased sucrose preference in rats following chronic unpredictable mild stress [59]. In another rat model of MDD that used chronic unpredictable stress, the TNF-α blocker infliximab, was also able to decrease immobility in the forced swim test [60]. Clinical trials have shown that immunomodulatory treatments decrease MDD symptomology. In a double-blind, placebo-controlled clinical trial, the NSAID celecoxib, given 200 mg twice daily for 6 weeks was shown to decrease levels of the proinflammatory cytokine IL-6 in patient serum along with decreased depressive symptoms as measured by the Hamilton Depression Rating scale [61]. Further, a meta-analysis of clinical trials investigating immunomodulators including NSAIDS, cytokine inhibitors, statins, minocycline, pioglitazone, and glucocorticoids on depressive symptoms showed that these pharmacologic agents improved patient symptoms. However, the authors conclude that future work should focus on long-term efficacy of anti-inflammatory drugs on MDD, as well as the inclusion of biomarkers to identify vulnerable groups of people that would benefit the most from such therapies [62]. It is important to note that limited studies have been conducted to explore the effects of immunotherapies on MDD and SUD/AUD comorbidities. While the immunomodulators discussed below target similar pathways and common abnormalities in cytokine levels and immunomodulators observed in both MDD and SUD/AUDs, preclinical and clinical studies addressing the effects of immunotherapies on these comorbidities are quite sparse and thus highlight the need for additional studies.

3.2. Bipolar disorder

Patients with bipolar disorder have been shown to be susceptible to SUD and AUD. Interestingly, males with bipolar disorder are more likely to experience SUD or AUD, with those experiencing more frequent manic episodes and suicidality being the most susceptible [63]. Others, however, have suggested that substance use is a predictor of bipolar disorder, highlighting substance use as a risk factor for developing bipolar disorder [64]. Regardless of its origination and/or causation, bipolar disorder is associated with elevations in proinflammatory cytokines and microglial immune activation, with evidence that elevations of some cytokines, including TNF-α and IL-6, could be related to disorder phase [65, 66]. In a randomized, double-blind, placebo-controlled clinical trial, the NSAID celecoxib was administered at 400 mg/day for 6 weeks, resulting in a higher remission rate for manic episodes that were unaccompanied by psychotic features [67]. Another clinical trial showed that the administration of 1 g/day of N-acetyl cysteine (NAC) as an adjunctive therapy for 24 weeks improved the depressive symptoms associated with bipolar disorder. NAC is a precursor of the antioxidant glutathione, which has downstream antioxidant and anti-inflammatory effects [68]. A meta-analysis of adjunctive treatment with anti-inflammatory agents such as NSAIDs, Omega-3 polyunsaturated fatty acids, NAC, and pioglitazone showed improved effectiveness of antidepressants when administered alongside anti-inflammatory medications. However, more studies with larger sample sizes are needed to determine treatment efficacy [69]. To date, there are no approved medications for targeting both bipolar disorder and SUD/AUD comorbidities. However, the use of some antipsychotics, such as D2/D3 receptor antagonists, as well as lithium have shown beneficial results in individuals with these comorbidities [70]. However, there is a current lack of evidence highlighting the use of immunomodulators for bipolar disorder/SUD/AUD comorbidities, and thus additional studies are warranted to explore their efficacies.

3.3. Anxiety

It has been well established that anxiety disorders and SUDs co-occur at rates greater than expected by chance alone [71, 72]. Interestingly, it has been suggested that these comorbidities arise from three different pathways: 1) anxiety disorder leads to an SUD (self-medication), 2) SUDs promote anxiety like states (substance-induced anxiety) or 3) a third variable pathway induced by genetics or anxiety. These pathways have been described in detail by Smith and Book [72]. More recent studies suggest that most individuals with this comorbidity fall into the second pathway of substance-induced anxiety, whereby substance use precludes the onset of generalized anxiety [71]. While comorbidities are high between the two, anxiety alone is known to induce a variety of neuroimmune alterations. Patients with anxiety disorders show elevated markers of inflammation including C-reactive protein (CRP), and several cytokines including IL-6, IL-1α, and IL-1β [73, 74]. Preclinically, the administration of the pro-inflammatory cytokine IL-1β increases anxiety-like behaviors as measured by the elevated plus maze and open field test that were blocked by co-administration of the anti-inflammatory cytokine IL-10 [75]. Immunomodulatory medications have also been investigated preclinically. Dexmedetomidine, and α2 adrenoceptor agonist, exhibited anti-anxiety effects as measured by the open field task as well as decreased NF-κB inflammatory signaling in a sleep deprivation model of anxiety [76]. Similarly, the TNF-α inhibitor etanercept decreased anxiety-like behavior observed in the elevated plus maze and depressive-like behavior observed in the forced swim test [77]. Clinically, immunomodulation for anxiety alone has yet to be thoroughly studied. Literature also remains sparce regarding the treatment of anxiety and SUD comorbidities. To date, the antioxidant N-acetylcysteine has been the only immunotherapy with promising effects on treating anxiety and SUD comorbidity [78]. Other treatments, including those discussed below, have not been explicitly studied for their effects on anxiety and SUD comorbidities, and highlight a major gap in our current understanding in treating anxiety/SUD comorbidities.

3.4. Schizophrenia

Up to 47% of patients with schizophrenia have been diagnosed with an SUD in their lifetime. There are several theories regarding the reason patients with schizophrenia suffer from comorbid SUDs, including hypotheses that schizophrenia and SUD share cognitive and genetic risk factors [79]. Furthermore, the comorbidity of schizophrenia and SUD is associated with poorer clinical outcomes in patients, having more severe implications in these individuals [80]. In terms of immune function, aberrant inflammation in patients with schizophrenia includes altered cytokine levels, complement signaling, innate and adaptive immune cell ratios and oxidative stress [81]. The Disc1-L100P mouse model of schizophrenia containing a point mutation in the DISC1^Rgsc139 gene exhibits alterations of lymphocyte cell and proinflammatory cytokine concentrations, suggesting a genetic influence over observed immune differences in patients with schizophrenia [82]. Clinical studies return mixed results. A double-blind placebo-controlled clinical trial showed that minocycline, an antibiotic medication described below, given to patients chronically treated with clozapine for schizophrenia and schizoaffective disorder did not improve positive symptomology but did improve working memory as well as symptoms of anxiety and depression [83]. However, a cross-country study showed that minocycline may decrease negative symptoms of schizophrenia when given with standard treatments [84]. Additionally, a double-bind placebo-controlled study showed that celecoxib, a COX-2 inhibitor, given with risperidone augmented the decrease in CD19+ B-cells and hastened the improvement of symptoms measured by the positive and negative syndrome scale by several weeks. Decreased soluble TNF-α receptor 55 predicted the extent of clinical improvement [85]. Interestingly, the antipsychotic drugs risperidone, quetiapine, clozapine, and haloperidol were shown to decrease IFN-γ in cultured cells from patients with schizophrenia [86]. This could explain the decrease anti-inflammatory action in both risperidone and risperidone + celecoxib groups even though celecoxib was showing to amplify these effects, underscoring the effectiveness of anti-inflammatory modulation on inflammation and behavioral outcomes. To date, pharmacological treatment for SUDs is underutilized in patients with schizophrenia relative to those without schizophrenia. There are currently no antipsychotic medications approved by the US food and drug administration (FDA) for SUD treatment in individuals with schizophrenia, however some of the antipsychotics discussed above have shown some efficacy [87]. For example, risperidone, aripiprazole and clozapine have each been shown to reduce symptoms of schizophrenia as well as SUD through modulation of serotonin and norepinephrine pathways [88–90]. To date, the immunotherapies described below have limited efficacy in regard to treating comorbid SUD and schizophrenia, however we believe this is due to lack of studies conducted as opposed to lack of efficacy. We believe that due to the overlapping immunological underpinnings of schizophrenia and SUD, the immunomodulators discussed below should be studied in the context of this comorbidity to fully understand their effectiveness in treatment.

4.1. Ibudilast

Ibudilast, first utilized in Asian countries to treat asthma [91], is a neuroimmune modulator shown to have anti-inflammatory effects within the central and peripheral nervous system. Ibudilast is known to selectively inhibit phosphodiesterase (PDE)-3, −4, −10 and −11, act as a TLR4 antagonist [92], and inhibit macrophage migration inhibitory factor (MIF) [93]. Although it is known to have multiple subcellular effects, the most well-known mechanism is its inhibitory effects on PDE, resulting in an increase in intracellular cAMP levels. PDEs are expressed within multiple human tissues and the CNS, where they play a role in hydrolyzing the phosphodiester bond of cAMP and cGMP, producing inactive AMP and GMP [94]. Physiologically, downregulation of cAMP and cGMP is hypothesized to result in downregulation of neurotrophic factors including nerve growth factors, neurotrophins as well as brain-derived neurotrophic factor. Not surprisingly, inhibition of PDE, as observed with Ibudilast, results in increased levels of cAMP, which has been shown to reduce nuclear factor kappa B (NF-κB), and subsequently reducing levels of pro-inflammatory cytokines and inducible nitric oxide synthase (iNOS) and reduced neuroinflammation [95]. It is also thought that the rise in cAMP levels induced by Ibudilast directly results in vasodilation and anti-thrombotic effects [96], rendering it advantageous for treatment of asthma. However, given recent preclinical evidence of its anti-inflammatory effects within the central nervous system, Ibudilast has become a desirable pharmaceutical for treating a variety of neurological disorders.

Ibudilast is currently being explored for its potential in treating a variety of diseases including multiple sclerosis, neuropathic pain, Alzheimer’s disease, and is currently undergoing clinical trials for its use in individuals with SUDs or AUDs. Preclinically, ibudilast has shown promising effects at reducing drug intake usage in rodent animal models. Specifically, it has been shown to reduce intravenous infusions of methamphetamine [97], reinstatement lever responding to methamphetamine [98], morphine induced conditioned place preference [99] and the amount of time required to extinguish drug seeking behavior for morphine [100]. In addition, ibudilast has been shown to reduce alcohol drinking in multiple animal models of AUD including both rats and mice. In these studies, exposure to alcohol vapor in alcohol preferring P rats, high alcohol drinking HAD1 rats, and mice followed by treatment with Ibudilast was shown to reduce alcohol consumption [101]. Such studies have demonstrated the need for clinical exploration of Ibudilast for treatment of SUDs and AUDs.

Clinically, ibudilast has been shown to reduce peripheral inflammatory effects following methamphetamine usage in individuals with methamphetamine use disorder, whereby ibudilast decreased methamphetamine-induced elevations in IL-6, sICAM-1, sVCAM-1 and cathepsin D compared to those given placebo [102]. While these studies have demonstrated changes in inflammatory markers in patients with SUDs following ibudilast treatment, studies assessing its effects on usage and abstinence remain unclear and relatively unexplored. For example, others have found that Ibudilast had no effect on methamphetamine abstinence in humans when given for 12 weeks at a dosage of 50 mg twice daily [103]. In addition, ibudilast has been shown to reduce withdrawal symptoms in opioid dependent individuals [104], yet had no effect on the subjective effects of oxycodone. Currently, ibudilast is being evaluated in a phase 2 clinical trial which is exploring the effects of ibudilast on neuroinflammation, brain function and connectivity in methamphetamine use disorder (Clinical Trial ID: NCT03341078). The phase 2 trial has 3 primary outcome measures, assessing the effects of ibudilast on neuroinflammation (assessed via positron emission tomography (PET)), brain function (assessed by magnetic resonance imaging (MRI)), and overall cognitive battery score. This clinical trial was set to end in December of 2023, however it has yet to be completed and results have not yet been made available.

Several clinical studies have examined the effects of ibudilast on alcohol consumption and associated reward circuitry. Interestingly, treatment of patients with AUD with ibudilast has been shown to reduce alcohol cue-elicited functional connectivity between orbitofrontal and anterior cingulate cortices, brain regions responsible for reward processing [105]. Further, this reduction in connectivity was correlated with a reduction in alcohol consumption [105]. Others also report that ibudilast reduces alcohol craving during stress exposure in individuals with AUDs [106], yet no molecular underpinnings were identified or explored. Similar results were reported by Grodin and colleagues [107], showing that ibudilast reduced heavy drinking in individuals with AUD, and demonstrated that high serum levels of C-reactive protein (CRP) could predict an individual’s responsiveness to ibudilast. In fact, individuals receiving ibudilast and had high serum levels of CRP had significantly fewer drinks per day than individuals receiving ibudilast yet had low serum levels of CRP [107]. Most recently, the effects of Ibudilast in AUD completed a phase 2 clinical trial, which ended in July of 2023. The clinical trial aimed to explore its effects on alcohol consumption in individuals with AUDs focusing on outcomes including percent of heavy drinking days (classified as 4+ drinks for women and 5+ drinks for men per day), percent of days abstinent, depressive symptomatology, and overall number of drinks per day (Clinical Trail ID: NCT03594435). The results of this trial have yet to be published.

Taken together, these preclinical and clinical studies demonstrate the promising therapeutic effects of Ibudilast on inflammatory markers, substance use and abstinence. Results from ongoing clinical trials will be advantageous to determine how this compound fairs on other beneficial markers and whether it will continue to prove advantageous for treatment in individuals with AUDs and SUDs.

4.2. Minocycline

Minocycline, a broad-spectrum antibiotic with antimicrobial, anti-inflammatory, antioxidant, and antiapoptotic actions, has been traditionally used to treat rheumatoid arthritis, dermatologic disorders, and sexually transmitted diseases. A second-generation tetracycline, minocycline has a longer half-life and increased lipophilicity than previous medications in this class, allowing for better blood brain barrier (BBB) penetrance [108, 109]. Its main mechanism of action as an antibiotic is to bind to the 30S bacterial ribosomal subunit, blocking bacterial mRNA transcription and protein synthesis. Anti-inflammatory actions include decreasing levels of cytokines like IL-1β as well as the endoproteases caspase-1 (cas-1) and caspase-3 (cas-3), which take part in several apoptotic and inflammatory actions including the synthesis of IL-18 and IL-1β through the formation and activation of inflammasomes [109]. Further, minocycline is thought to be neuroprotective due to its ability to decrease the number of immune activated microglia, scavenge free radicals to decrease oxidative stress, and to restore BBB integrity through the inhibition of matrix metalloproteases (MMPs), which can also lead to a reduction T-cell infiltration [109, 110].

Several preclinical studies suggest that minocycline may produce beneficial behavioral and anti-inflammatory results. The administration of minocycline blocked the maintenance of methamphetamine-induced conditioned place preference and facilitated extinction of drug seeking behaviors in a rodent model of meth use disorder [111]. Conditioned place preference was likewise disrupted by minocycline pretreatment in a rodent model of cocaine use disorder, along with decreased microglial immune activation [112]. Other rodent models of substance use investigated the ability of minocycline to reduce the impact of risk factors such as depression and traumatic brain injury on drug consumption. Minocycline increased levels of the antioxidant glutathione following methamphetamine use. This corresponded with decreased depressive-like symptoms as measured by the forced swim, tail suspension, and sucrose splash tasks [113]. In another study utilizing a two-bottle choice task, traumatic brain injury-induced increased alcohol intake was attenuated by minocycline administration. This corresponded with lowered levels of the pro-inflammatory cytokine IL-1β [114]. Lastly, minocycline has been shown to attenuate opioid withdrawal symptoms in rats, which was associated with reductions in pannexin-1 and p-mTOR expression of microglia [115]. A similar study has also shown that inhibition of microglial activation and downregulation of CaMKII, Ras and p-ERK attenuated morphine withdrawal [116]. These findings suggest that minocycline has the ability disrupt addictive and addictive-associated behaviors, with possible mechanisms relating to decreased oxidative stress and attenuation of proinflammatory cytokines.

Clinically, minocycline has produced mixed results in patients with SUDs. Whether minocycline affects drug craving could be drug-type specific. In patients with opioid use disorder, minocycline failed to produce changes in opioid craving, symptoms of withdrawal, or serum cytokines but did increase accuracy on a Go/NOGO task [117]. Additionally, minocycline did not impact serum cytokine levels, subjective positive effects of alcohol, or alcohol craving in heavy drinkers [118], but did produce an age-dependent difference in cognitive tasks. A study of recreational opioid use reported that participants observed a decrease in the positive and liking effects of oxycodone, though it’s analgesic and physiological effects remained intact [119]. Though these studies suggest that minocycline might be cognitive enhancer in patients with SUDs, leaving craving and serum cytokines largely unchanged, these studies are underpowered to produce definitive results.

Currently, minocycline is in phase 1 and 2 clinical trials for AUD, with combined imaging alongside craving and cognitive assessments. In one such trial, participants are treated in a randomized, double-blind, placebo-controlled study assessing the immune activation of microglia cells using positron emission tomography (PET) with radiotracers for N-(2,5-dimethoxy-benzyl)-N-(5-fluoro-2-phenoxyphenyl) acetamide, labeled with carbon-11 ([¹¹C]-DAA1106) to trace immune activated microglia. Serum cytokine levels were also assessed. Alongside markers of immune activity, participants underwent a battery of neurocognitive and addiction-related assessment to determine craving, working memory, cognitive flexibility, along with other cognitive tasks. The trial has concluded (January 2019) but results are yet to be reported (Clinical Trail ID: NCT03244592). Another phase 1 clinical trial for AUD is underway, similarly assessing several cognitive tasks, alcohol usage, as well as serum cytokine levels. Additional assessments include magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) and gut microbiota evaluation (Clinical Trail ID: NCT04210713).

Minocycline appears to decrease microglial immune activation, exert antioxidant effects, and decrease pro-inflammatory signals, with behavioral therapeutic effects potentially mediated through decreasing the effects of risk factors such as comorbid conditions such as depression and TBI. The mixed results in patient use of minocycline appear to be drug type specific, with the largest benefits appearing to be related to cognitive processes. Current clinical trials aim to address the current lack of human imaging data corresponding to serum immune cytokines and cognitive/behavioral effects in patients with AUDs treated with minocycline. Though minocycline’s usefulness to treat SUDs is promising, future clinical trials are needed in addition to the current alcohol-related trials for a better understanding of the effects minocycline treatment has on disorders related to different drugs of abuse.

4.3. Probenecid

Probenecid is a uricosuric agent that was initially created to reduce renal excretion of penicillin but is more notable for its treatment of chronic gout [120]. In this context, the drug competitively inhibits organic anion transporters (OAT) in the kidney, specifically OAT1 and OAT3, which decreases uric acid reabsorption and increases urine excretion of urate [121–123]. Probenecid was also found to inhibit acidic monoamine metabolites moving from the central nervous system (CNS) to the blood by similarly suppressing OATs [120]. As a result, probenecid was utilized in studies that hypothesized differences in monoamine metabolite concentration could be diagnostic for depression [124]. More recently, the drug has been investigated for its ability to modify the activity of transient receptor potential vanilloid 2 (TRPV2) channels and pannexin 1 (Panx1) channels [125]. Probenecid is an agonist of TRPV2 channels which are transiently expressed in tissues throughout the body, including the heart [120]. TRPV2 activation is heavily implicated in sensation and nociception, and probenecid is proven to activate this channel in a highly specific manner [126]. In addition, the drug inhibits Panx1 channels which play vital roles in neurotransmission and neuroinflammation [127]. Panx1 channel activity is broadly activated by substrates and is highly associated with ATP secretion [128]. Within brain cells, high amounts of extracellular ATP subsequently activate P2X and P2Y receptors, triggering a neuroinflammatory response [129]. Despite having multiple cellular effects, the drug’s ability to selectively inhibit OAT and Panx1 activity are of particular interest in relation to substance abuse.

Probenecid has demonstrated promising preclinical results for treatment of AUDs and opioid withdrawal alleviation. Its potential intervention in alcohol consumption was investigated after carbenoxolone was observed to reduce alcohol intake in rodents by a mechanism that included inhibition of Panx1 channels [127]. Similarly, probenecid’s inhibitory effect on Panx1 channel activity has been shown to reduce alcohol intake in rodents in both alcohol dependent and alcohol independent groups [127]. This study distinguished the reduced intake to be independent from a reward-based decision pathway. Another study focused on utilizing probenecid’s inhibition of OAT channels to prolong the action of Ro 61–8048, a competitive inhibitor of kynurenine-3-hydroxylase [130]. This inhibitory effect resulted in an accumulation of kynurenine in the brain, which was shown to significantly decrease consumption of ethanol in rodents in both sexes [130]. The drug has also shown efficacy in reducing the severity of withdrawal symptoms of opioids with Panx1 channel as the therapeutic target. Despite being administered at various intervals times in relation to morphine dosage, Probenecid consistently suppressed ATP release and concurrently minimized withdrawal symptoms in rodents. This led to the conclusion that normal Panx1 activity is required for morphine withdrawal [131]. The drug’s efficacy in modulating substance abuse behavior and mitigating opioid withdrawal symptoms offer promise in clinical investigation.

Investigations using probenecid in human trials for alcohol abuse and opioids withdrawal are in progress. Despite the absence of published evidence, the drug continues to be an attractive candidate for repurposing, as it is readily accessible and has a history of being well-tolerated in human patients [125]. It currently is in phase 1 studies for AUD (Clinical Trial ID: NCT04218357). This randomized, double-blind trial administered an oral dose of 2 grams of probenecid or placebo during 2 different sessions. The goal of this study is to test the efficacy and safety of the drug when administered to individuals with AUD as defined by the researchers’ criteria. The drug’s clinical investigation for alleviation of opioid withdrawal is further along in development, with a phase 2 study currently in progress (Clinical Trial ID: NCT04939623). This 12-week clinical study contains 3 groups given treatment twice a day: probenecid 500 mg, probenecid 1000 mg, and placebo. The study primarily aims to evaluate the safety of oral probenecid in individuals voluntarily planning to reduce opioid intake. A relevant secondary goal is to investigate if genetic expression of different Panx1 alleles affect therapeutic outcomes of probenecid.

Although preclinical results regarding probenecid’s impact on AUDs and opioid withdrawal are encouraging, definitive conclusions for their efficacy in humans must wait for the results of current ongoing clinical trials. While current literature describes the effects of probenecid at Panx1 as the central therapeutic target, others have shown differing structure between humans and mice. For example, Panx1 activation responses in humans are insensitive in the presence of high extracellular potassium which has yet to be shown in mice [132]. Thus, discrepancies in both function and structure between species open the door to varying clinical effects in the utilization of probenecid for substance abuse treatment in humans and further exploration into these differences is essential.

4.4. Dexmedetomidine

Dexmedetomidine was originally approved in 1999 as a perioperative sedative for use in intensive care units (ICUs) but has recently been explored off label as a central and peripheral neuroimmune modulator [133]. Like clonidine, dexmedetomidine is an α2 noradrenergic agonist but is more selective for the α2 receptor over the α1 receptor, conferring a 1620:1 preference as compared to 220:1 for clonidine [133, 134]. Unlike barbiturates and intravenous anesthetics like propofol that exert action on the gamma-aminobutyric acid (GABA) system which can lead to respiratory depression, it can be given with minimal respiratory consequence. Importantly, dexmedetomidine is FDA approved for the treatment of schizophrenia and bipolar I and II disorders. A phase 3 randomized, double-blind placebo-controlled clinical trial showed that 2 hours post-administration, dexmedetomidine reduced acute agitation in patients with schizophrenia as measured by the Positive and Negative Syndrome Scale-Excited Component (PEC) [135].

The main target of dexmedetomidine, α2 receptors, exert an inhibitory effect at pre-synaptic autoreceptors by decreasing calcium influx as well as postsynaptically by decreasing intracellular cAMP levels, increasing activity of inwardly rectifying potassium channels, and hyperpolarizing the cell. Due to these effects, dexmedetomidine produces a sleep-like state from which the subject can be relatively easily awoken [133]. Dexmedetomidine acts as an immunomodulator by decreasing intracellular NF-kB signaling, leading to a suppression of pro-inflammatory cytokine synthesis and release. This leads to the decrease in cytokines such as TNF-α, IL-6, and IL-8, as well as a decrease in the phagocytotic activity of macrophages. Evaluation of patient post-surgery plasma reveals an increased ratio of Th1 to Th2 cells. Additional indirect immunomodulatory effects include decreasing the activity of the hypothalamic-pituitary-adrenal (HPA) axis, a neuroendocrine signaling pathway that modulates the stress response and which contains targets (pituitary, adrenals) expressing α2 receptors. This decrease in HPA activity decreases cortisol release, which has been shown to modulate the distribution and movement of leukocytes [134]. Preclinically, the immunomodulatory effects of dexmedetomidine have not been thoroughly investigated in the context of drug use, behaviorally or molecularly. However, in a mouse model of sevoflurane anesthesia-induced post-operative cognitive dysfunction, dexmedetomidine decreased hippocampal damage by regulating microglial NF-κB signaling [136]. More research is needed to determine dexmedetomidine’s ability to modulate immune signaling and behavioral outcomes following substance use.

Clinical trials are underway to investigate the effectiveness of dexmedetomidine for the treatment of withdrawal symptoms. Sublingually delivered dexmedetomidine (therein named BXCL501) was investigated in a phase 1B/2 clinical trial to determine its safety, pharmacokinetics, and early efficacy for the treatment of opioid use disorder. The primary outcome measure were symptoms of withdrawal after dexmedetomidine treatment following the cessation of morphine maintenance therapy using the Clinical Opiate Withdrawal Scale (COWS) and Short Opiate Withdrawal Scale (SOWS) (Clinical Trial ID: NCT04470050). Published findings suggest that dexmedetomidine is well tolerated and decreased physiological symptoms of withdrawal like resting pulse rate, sweating, pupil size, and irritability as measured by the COWS, along with decreased subjective symptoms of withdrawal such as “feeling sick” and “heart pounding” ratings as measured by the COWS [137]. Another phase 1 clinical trial that was recently completed (2/2023) investigated the effects of sublingual dexmedetomidine on alcohol cue reactivity and stress in patients with comorbid AUDs with PTSD. Results have not been published (Clinical Trial ID: NCT04827056). One other phase 1 clinical trial is currently recruiting participants with the goal of investigating the efficacy of dexmedetomidine on cigarette smoking behavior, symptoms of withdrawal, and craving. Along with dexmedetomidine, this trial will also explore potential benefits of the NMDA antagonist ketamine and the GABA_A positive allosteric modulator midazolam (Clinical Trial ID: NCT05505630).

The immunomodulatory effects of dexmedetomidine appear to be both direct via decreased pro-inflammatory cytokines and indirect through modulation of the stress response. These indirect effects are likely due to its action as a sedative, leading to decreased HPA-axis stimulated glucocorticoid release and decreased sympathetic nervous system mediated symptoms of withdrawal. Future work should focus on determining the role of these different but corresponding potential mechanisms that could underlie the potential for use of dexmedetomidine in treating SUDs.

4.5. Pioglitazone

Pioglitazone, a thiazolidinedione (TZD) drug for type 2 diabetes, is prescribed either as a standalone intervention or in conjunction with other medication. The general therapeutic effect of pioglitazone increases insulin sensitivity in liver, adipose and skeletal muscle tissue [138, 139]. On a molecular level, the drug is a potent agonist of the gamma form of peroxisome proliferator-activated receptors (PPARγ) [140]. These proteins are nuclear transcription factors that regulate adipocyte gene expression and proper cell development [141]. PPARγ also reduce lipid overproduction in non-adipose tissue and modulate secretion of adiponectin and leptin [142]. Agonists of PPARγ, such as pioglitazone, demonstrate anti-inflammatory and potentially anti-carcinogenic properties, but their intervention in cardiovascular disease is convoluted [143, 144]. TZDs have a cardioprotective effect in both diabetic and nondiabetic patients due to their ability to stunt the development of atherosclerosis. This effect is the hypothesized mechanism underlying an association of TZDs with decreased risk of myocardial infarction and stroke [145, 146]. Contentiously, rosiglitazone, a specific TZD, was associated with increased incidence of congestive heart failure, acute myocardial infarction, and death in elderly diabetic patients [147]. Additionally, pioglitazone has raised potential concerns of fluid retention, weight gain, and heart failure in insulin-resistant and diabetic patients [148]. It is worth noting that all TZDs are linked to oedema and fluid retention, which are non-discriminating signs of heart failure [145, 149]. Despite these concerns of adverse secondary effects, researchers remain confident that pioglitazone has fewer unintended complications than rosiglitazone and lacks direct associations to heart complication [150]. Recently, pioglitazone’s agonistic effect on PPARγ has been investigated in relation to its potential attenuation of addictive-like behavior in laboratory animals.

Pioglitazone has been extensively applied preclinically due to PPARγ expression on glial cells in areas of the brain highly implicated in reward and addiction pathways, such as the nucleus accumbens [151]. The drug has demonstrated an inhibitory effect on neuroinflammation that is likely through the reduction of cytokine release by microglia [152]. This is of particular interest in alcohol abuse where ethanol usage proliferates an inflammatory response from microglia with associated neuroinflammatory cytokines [153]. In mice, pioglitazone significantly reduced alcohol consumption and alcohol seeking, a proposed behavior in relapse [154]. The researchers hypothesized that pioglitazone could have similar mechanistic and therapeutic effects as ibudiblast, a drug that achieves the same reduction of microglia-mediated inflammation through TLR inhibition, as discussed above [154, 155]. In addition, pioglitazone decreased binge-drinking in mice, a more nuanced behavior believed to parallel excessive alcohol abuse in humans [156]. The drug has also been studied preclinically as an intervention in both natural and synthetic opioid abuse. Glial cell response to opioids and subsequent neuroinflammatory response were initially proposed to play a key role in developing a morphine tolerance [157]. This was shown as pioglitazone not only inhibited the development of tolerance when administered concomitantly with morphine, but also reversed effects when given after the tolerance was established [158]. The drug has similarly been demonstrated to reduce heroin self-administration, stress-induced heroin seeking behavior, and potentially heroin-relapse behavior in mice [158, 159]. Furthermore, pioglitazone’s preclinical therapeutic use extends to concomitant administration with stimulants such as methamphetamine and cocaine. In mice, methamphetamine interestingly upregulated PPARγ activity, but subsequent intervention with had pioglitazone had an inhibitory effect on the development of behavioral sensitization [160]. This pattern of behavior is an established effect of psychostimulants in rats that is believed to replicate the human psychoses triggered by psychostimulants [161]. Pioglitazone similarly has been shown to reduce behavioral sensitization to cocaine as well as lessen drug-seeking and relapse behavior [162]. Thorough examination of pioglitazone preclinically highlights its diverse potential as an intervention to modulate behavior clinically.

Accordingly, pioglitazone has been a subject of interest for human patients suffering from AUDs. In treating AUD in clinical trials, pioglitazone has been met with the same controversies previously described. In 2020, researchers detailed their preliminary cancellation of their phase 2 trial using pioglitazone at 45 mg a day (NCT01631630). This was due to the concurrent publication of research questioning the safety of TZDs as well as elevated levels of creatine kinase in the trial participants. The researchers of the concurrent study also suggested that pioglitazone surprisingly increased alcohol cravings [163]. Although interesting, this was likely a result of a small participant pool and limited duration of study. In 2021, a chart review revealed that pioglitazone significantly reduced Alcohol Use Disorder Identification Test: Consumption (AUDIT-C) scores in individuals prescribed the drug for diabetes [164]. As more recent studies confirm pioglitazone’s safety in administration, the drug has since been approved for additional clinical trials. These include ongoing phase 2 studies on pioglitazone’s effect on stress reactivity and alcohol craving (NCT05107765) as well as for AUD (NCT03864146). Notably, a trial regarding pioglitazone’s effect on alcoholic lung immune defense (NCT03060772) was interrupted due to COVID and never resumed. For clinical investigation of pioglitazone’s use for opioid related disorders, the drug fails to replicate preclinical promise. Studies have shown that pioglitazone has no effect on reducing opioid withdrawal symptoms or the abuse liability of both oxycodone and heroin [165, 166]. The most promising finding was the drug’s ability to reduce heroin craving, but the absence of supportive research has led to a scarcity of continued clinical investigation [167]. Perhaps the most promising clinical investigation of pioglitazone is its application for cocaine use disorders (CUD). Participants with CUD were administered pioglitazone 3 times a week for 12 weeks and displayed not only reduced cocaine craving, but also improved white matter integrity [168]. The latter has been proven to be related to increased impulsivity in cocaine patients, whereby improved integrity reduces impulsivity [169]. Currently, a study is being conducted regarding pioglitazone’s ability to prevent relapse in cocaine users with a concurrent cognitive behavioral therapy treatment (NCT04843046).

Despite pioglitazone’s lack of direct translation of preclinical to clinical success, the drug remains a potentially promising subject of interest in treating SUDs. Not only has it been associated with positive intervention in mainly cocaine and alcohol use disorders, but also is being investigated for other uses, including a phase 2 trial for prevention of oral cancer (NCT05727761). But the challenge of translating preclinical results in rodents to humans will remain for pioglitazone. Although research has found way to create more human-like alleles of PPARγ in rodents, the metabolic differences in how these species incorporate the drug into disease processes will continue to generate discrepancies [170]. Additionally, it is imperative to consider the conflicting results previously presented regarding pioglitazone’s safety. Although the drug has shown “favorable risk-benefit analysis”, its history of association with adverse secondary effect should not be ignored [171].

4.6. Cannabidiol

The medicinal application of the cannabis plant has been well documented throughout history. One of its many derivatives, cannabidiol (CBD), has been a subject of interest in medical research since its isolation in 1963. CBD is frequently discussed in relation to another 21-carbon phytocannabinoid, tetrahydrocannabinol (THC) [172]. THC is known for its “intoxicating” effects on psychomotor function which leads to the sensation of the reported “high” [173]. On the other hand, CBD is considered non-intoxicating and has a safer profile. As a result, CBD has been investigated for a wide range of therapeutic applications that include reducing inflammation and treating gastrointestinal and neural disorders [174]. The derivative is also a proposed intervention in substance abuse disorder largely due to its ability to modulate a variety of receptors in the endocannabinoid system (ECS). Cannabinoid type 1 and 2 receptors (CB1R, CB2R) are key G-protein coupled receptors in the ECS that are negatively and allosterically modulated by CBD [175]. CB1R is one of the most widely expressed GPCRs in the CNS and has been a central target for treating neural disorders [176]. CB2R is also a prime target for treating addictive disorders as it is has been found in reward centers of the brain [177]. Furthermore, CBD has a wide range of targets outside of the CBRs. Of notable interest is the drug’s action on mesolimbic dopamine transmission. For example, CBD has demonstrated that it can normalize dysregulation of dopaminergic neuron activity [178]. It also has an agonistic effect on TRPV receptors whose activity is implicated in neuroprotection [174]. Another application for CBD is its ability to stimulate PPARγ which have a high level of expression in the nucleus accumbens [175]. Similar to pioglitazone, CBD excites PPARγ in a mechanism that may alleviate negative symptoms associated with addiction, such as withdrawal [175]. CBD’s established effect on receptor targets with implication in substance abuse disorders have made the cannabis derivative a prime suspect for further investigation.

CBD’s wide range of potential therapeutic applications has led to an extensive preclinical investigation for treatment/therapeutic effects on SUDs. The drug has had success in addressing both physiological and behavioral facets of AUDs in rat models, partly due to its anti-inflammatory effects. Specifically, CBD has an anti-inflammatory effect which inhibits alcohol’s stimulation of immune cells and their associated inflammatory cytokines [179]. Additionally in rats, CBD has demonstrated an ability to counteract the neurodegeneration and cell death in the hippocampus precipitated by alcohol abuse [180]. The drug’s alteration of behavioral response to alcohol is likely due to its negative modulation of CB1R. In rats, CB1R inhibition has been consistently shown to reduce ethanol consumption [181, 182]. By reducing CB1R activity, CBD decreases ethanol consumption, relapse behavior after sensitization, and binge-drinking behavior after repeated CBD administration [183]. Moreover, CB1Rs are a subject of interest in regulating opioid reward, as the absence of these receptors lead to decreased opioid self-administration in mice [184]. Through negative modulation of CB1R, CBD not only enhances the nociceptive properties of morphine in mice, but also attenuates morphine withdrawal symptoms [185, 186]. This dual action could reduce the initial therapeutic dosage for pain management and moderate the behavior associated with termination of treatment. CBD exhibits a similar preclinical effect on heroin administration in mice, reducing withdrawal behavior when heroin administration is abruptly terminated [187]. Furthermore, CBD has displayed an ability to modulate the effect of stimulants such as methamphetamine and cocaine. In rats, CBD inhibited dopamine uptake during co-administration with methamphetamine leading to a reduction in locomotor activity, reversal of behavioral sensitization, decrease of desire to self-administer, and potentially inhibition of methamphetamine-induced psychosis [188, 189]. Similarly, CBD is shown to alter the effects of cocaine through modification of dopaminergic pathways. This has also been hypothesized to cause notable behavioral changes in rats that include a reduction in the rewarding effects of cocaine and alleviating withdrawal symptoms [190, 191]. CBD’s diverse potential in SUDs has sparked an expansive interest in clinical testing in humans.

CBD’s appeal as an intervention in humans stems from its lack of abuse potential and adverse side effects. In the past, CBD has been shown to reduce levels of blood alcohol when administered simultaneously but failed to produce notable effects on behavioral activity [192]. Recently, two clinical studies were conducted for the drug’s potential effect on alcohol, with the first examining its effects on blood alcohol level and resulting intoxication (NCT06105138). Preliminary data has not proven an ability to significantly alter breath alcohol or behavior suggestive of intoxication. However, a phase 2 clinical trial examining CBDs effects on alcohol consumption and craving has recently been completed but data have yet to be reported (NCT04873453). Additionally, studies regarding youth alcohol and CBD’s intervention have begun recruitment for a phase 1 clinical trial. In treating opioid addiction, CBD has demonstrated success in reducing cue-induced craving and anxiety in individuals with heroin use disorder. CBD also had physiological effects that included limiting drug cue induced heart rate and salivary cortisol levels [193]. This promise has led to continued research in CBD’s intervention in opioid addiction. One phase 2 study currently aims to examine if CBD can inhibit cue-induced craving and anxiety in patients taking methadone or buprenorphine for opioid use disorder (NCT06206291). Another phase 2 study is using CBD to examine its potential to decrease craving and reduce withdrawal behavior (NCT03787628). Regarding the treatment of stimulant use disorders, CBD has not replicated its preclinical success within the clinical setting. It has shown no ability to alleviate crack cocaine withdrawal [194], or improve cognitive outcomes in individuals with cocaine use disorder [195]. As a result, there is little evidence of ongoing trials regarding CBD’s application clinically for stimulants.

Like the other drugs examined, CBD exhibits a disconnect between preclinical results and clinical efficacy in humans for substance abuse disorder. As mentioned previously, the drug has a wide range of targets on a cellular level likely contributing to the discrepancy in results observed between preclinical rodent models and humans. Despite this, CBD appears to have promise in its effect on alcohol abuse and opioid abuse. Additionally, studies suggest that CBD can reduce addictive behavior of cannabis use, offering another potential application [196]. Furthermore, CBD has demonstrated a wide range of therapeutic options, such as pain relief and intestinal inflammation. Combined with its low abuse potential in individuals taking multiple medications, CBD remains a promising therapeutic and warrants continued research efforts for treatment of SUDs [197].