The Potential of Pharmacogenomics to Treat Drug Addiction

Pharmacogenetics & personalized prescribing

Tailoring pharmacotherapy to an individual‘s genetic makeup to optimize treatment efficacy, while decreasing toxicity and adverse reactions seemed like science fiction only a few years ago. Genomic technology has advanced to allow personalized prescribing for many medications. Indeed, the US FDA‘s list of valid genomic biomarkers has expanded to include many SNPs and other gene variants that affect drug efficacy through altered pharmacokinetics and pharmacodynamics [1]. Incorporation of gene-array assay technology into mainstream clinical medicine is enhancing patient care. Pharmacogenomic biomarkers include cytochrome P450 hepatic enzyme variants that affect drug metabolism for most psychiatric medications [2,3]. Drugs that target serotonergic, noradrenergic and dopamine systems in psychiatric illness all have pharmacodynamic-associated gene variants that can predict therapeutic response and serious adverse drug reactions [4–8].

We previously reviewed evidence suggesting pharmacogenetics may be used to individualize drug dependence treatments [9,10]. Here are a few examples of medications for drug dependence disorders for which pharmacogenetic know-how may be useful to increase efficacy.

Alcohol dependence: naltrexone & topiramate

The µ-opioid antagonist naltrexone for alcohol dependence is probably the most extensively researched pharmacogenetic association to date. An asparagine-to-aspartate amino acid substitution at position 40 (Asn40Asp) in the µ-opioid type 1 (OPRM1) receptor gene changes the binding of the primary endogenous ligand β-endorphin to the µ-receptor [11–13]. Subjects with this SNP report altered alcohol subjective effects, and have a greater cortisol response to naltrexone [14,15]. Three clinical trials show that those harboring the Asp40 allele are over three-times more likely to respond to naltrexone therapy for alcohol dependence compared with those homozygous for the Asn40 allele [16–18].

Topiramate is an anticonvulsant that is efficacious for alcohol dependence [19–22]. It may decrease alcohol craving by facilitating GABA while simultaneously inhibiting glutamatergic neurotransmission [23]. However, topiramate-induced paresthesia, fatigue, somnolence, hypoesthesia and nausea in some patients have limited its widespread use. Interestingly, Ray et al. identified a SNP in intron 9 of the glutamate receptor GluR5 gene (GRIK1) that appears related to topiramate-induced side effect severity in alcohol abusers, thereby suggesting a genetic marker to potentially determine patients poorly suited for this medication due to these side effects [24].

Pharmacogenetics of methadone maintenance & buprenorphine for opiate dependence

The opioid agonist methadone and partial agonist buprenorphine are successful replacement therapies for opiate dependence [25]. Both synthetic opioids are affected by CYP-regulated pharmacokinetics that influence their treatment efficacy [26]. Methadone and buprenorphine are both metabolized by CYP3A4, but buprenorphine is metabolized substantially less than methadone by CYP2D6 [27,28]. There have been difficulties in managing CYP2D6 ultra-rapid metabolizers taking methadone once-daily and these patients may be more optimally managed on buprenorphine for addiction treatment [29]. A rapid hepatic SNP screen is already available to help select these ultra-rapid metabolizers for buprenorphine rather than methadone pharmacotherapy.

Dopamine D2 receptors play a significant role in the reinforcing effects of many drugs of abuse, and are decreased in opiate-dependent subjects [30–33]. Interestingly, a recent study showed that patients who have the (DRD2) 957C>T polymorphism respond poorly to methadone maintenance treatment, again suggesting that this population is better treated by buprenorphine than methadone [34].

Nicotine replacement therapy, buproprion & varenicline

Nicotine influences smoking behavior and is metabolized by CYP2A6 to its inactive form cotinine. Subjects with the CYP2A6 ultra-rapid metabolizer genotype smoke more and are less likely to quit smoking, whereas those that have alleles associated with slow nicotine metabolism smoke fewer cigarettes per day and have lower risk of developing lung cancer [35–37]. This variation in metabolism also affects the efficacy of FDA-approved nicotine replacement therapy, and possibly the α4β2 nicotinic receptor partial agonist varenicline. Slow metabolizers using nicotine replacement reach higher plasma nicotine levels compared with normals and thus need less of the medication, which can reduce side effects and cost, and increase adherence and efficacy. An important new question is whether these pharmacogenetically selected patients might also do particularly better with varenicline than with nicotine replacement therapy [38,39].

Cocaine dependence: disulfiram, dexamphetamine & methylphenidate

Although there are presently no FDA approved pharmacotherapies for stimulant addiction, the dopamine-β-hydroxylase inhibitor disulfiram [40,41], and the attention deficit–hyperactivity disorder medications dexamphetamine [42–45] and methylphenidate show therapeutic promise in genetically selected subgroups of patients [46]. In both cases the relevant neurotransmitter system is dopamine. From a pharmacogenetic perspective, the functional C-1021T SNP results in significantly lower circulating DβH levels and alternative pathways of converting dopamine to norepinephrine. This relatively common abnormality (∼40% of population) has been associated with lack of efficacy for disulfiram in cocaine addiction [47]. Individuals with the DAT1 genotype 3´-untranslated variable number tandem repeat polymorphism 9/9 repeats do not experience the euphoric or physiologic effects of dextroamphetamine [48]. Patients that have this polymorphism also experience higher rates of side effects from methylphenidate than attention deficit–hyperactivity disorder patients without this polymorphism [49,50]. Taken together, both of these polymorphisms, which affect dopamine metabolism and pharmacodynamics respectively, are promising candidates for improving response to these medications for stimulant abuse.

Overall, pharmacogenetics is rapidly finding a place in drug addiction pharmacotherapy in order to optimize matching of patients to our evolving new treatments.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.