Our findings showed that ondansetron is a promising therapeutic agent for the treatment of severe alcohol consumption among alcohol-dependent individuals with the LL genotype of the 5′-HTTLPR. These findings are supported by a pilot human laboratory study where ondansetron suppressed alcohol self-administration in the LL compared with the LS/SS genotype significantly among non-treatment-seeking alcoholics (46
From a qualitative clinical perspective, the relationship between drinking level and health consequences is not linear. For men, who constituted 73% of our subject population, high-risk drinking of alcohol, which occurs at or above 5 drinks/drinking day, is associated with severe health consequences, including accidental injuries (47
), deaths from external sources (48
), being a target of aggression or committing an aggressive act (49
), and numerous medical, legal, and occupational problems (50
). Therefore, using that clinical criterion, subjects with the LL or LL/TT genotype treated with placebo remained in the high-risk drinking category. In contrast, subjects with the LL or LL/TT genotype treated with ondansetron were, on average, no longer in the high-risk drinking category. Thus, subjects with the LL or LT/TT genotype who received ondansetron, compared with their counterparts who got placebo, had a qualitatively greater clinical improvement. Furthermore, the clinical importance of our findings was underscored by demonstrating that both LL and the combination of LL/TT genotypes also were significant predictors to increasing the percentage of days abstinent for those who received ondansetron compared with placebo.
Interestingly, the concomitant possession of the TT genotype of rs1042173 among those with the LL genotype of the 5′-HTTLPR enhanced the treatment response to ondansetron. Obviously, the magnitude of this increased effect cannot be determined directly within the same individual because the LL and TT genotypes are located within the same 5-HTT gene. It can, however, be inferred indirectly from the finding that the treatment effect of ondansetron among those with the combined LL and TT genotypes is significantly greater than that of an undifferentiated group of those with the LL genotype who would have different allelic frequencies of the TT, TG, or GG genotype, and the combined LL/TT group that received ondansetron had the best outcome. Nevertheless, despite our finding of a statistical interaction between the LL and TT genotypes, the exact molecular mechanism for this added therapeutic effect remains to be determined.
The operative issue in the clinical practice of pharmacogenetics is to identify and treat those who will respond the best to a particular medication. Hence, our findings promulgate the clinical approach of identifying alcohol-dependent individuals who are likely to respond to ondansetron based on their genotype analysis at the 5-HTT gene (i.e., LLs with or without the TT genotype) and then providing them with the medication.
Importantly, our primary outcome variable tested a specific scientific hypothesis related to ondansetron’s effects on the severity of alcohol consumption in alcohol-dependent individuals who varied by 5-HTT genotype. This specific pharmacogenetic hypothesis was proposed prior to the conduct of the present study (4
), and developed through systematic experimental studies (3
). Testing a specific hypothesis is a conservative scientific approach because it avoids the presumption that a particular biological or genetic effect can be extrapolated to explain different patterns of drinking behavior. Consistent with this approach, we restricted the test of the effect of ondansetron response to just one other secondary variable—percentage of days abstinent—to provide additional information to clinicians. Our findings underscore the clinical importance of our results because they demonstrate that both LL and the combination of LL/TT genotypes predicted therapeutic response to ondansetron by increasing the percentage of days abstinent relative to placebo.
Notably, even though we enrolled into this clinical trial currently drinking alcohol-dependent individuals who had to have a breath alcohol concentration of no greater than 0.02% for clinical attendance, the breath alcohol concentration level reported in the trial was extremely low, and no significant withdrawal symptoms were reported. This phenomenon shows that alcohol-dependent individuals are able to moderate or taper their drinking level, perhaps as a result of the behavioral contingency of monitoring their drinking level (51
). Also, this finding demonstrates the feasibility of providing treatment to alcohol-dependent individuals at the typical point of maximum crisis—when they are actively drinking alcohol and asking for help—and suggests that prior detoxification is not always a necessary prerequisite for the treatment of alcohol-dependent individuals who can be managed within an outpatient setting.
Our study was limited by five factors. First, we did not have equal numbers of individuals with different ethnicity. Hence, we could not test formally for a specific effect of ethnicity on genetic profile; however, the general trend and magnitude of the differences reported were similar for Whites and Hispanics (data not shown). Additionally, we performed an analysis for genetic structure with 24 unlinked ancestry-informative markers, which has been reported to be as effective in estimating continental population ancestry as are analyses with more than 90 ancestry-informative markers (38
), and these ancestry proportion estimates were used as covariates in all our statistical models in place of self-defined ethnicity. The ancestry proportions, estimated by the program STRUCTURE in each of the three parental population groups, were symmetric between samples in the ondansetron and placebo groups (data not shown). Furthermore, we did not find in our statistical analyses that ancestry proportions were a significant covariate. Second, not all alcohol-dependent individuals were treated successfully with ondansetron—just those with a specific allelic constitution of the 5-HTT gene. Hence, more research is needed to find alcoholics with other genetic polymorphisms who will respond significantly to alternative medications. Third, there might be other genetic variants within the 5-HTT gene that could increase further the specificity of ondansetron’s treatment response; such relationships can only be elucidated through larger-scale pharmacogenomic studies. Fourth, the molecular mechanisms associated with the differential effect in the severity of alcohol consumption on L and S variants of the 5′-HTTLPR and T and G alleles of the 3′-UTR are currently not well understood; thus, additional in vitro
and in vivo
studies are needed. Fifth, whilst we considered the possibility that comorbid depression or anxiety-related disorders that have shown an association with serotonergic genotypes could confound our findings, we think this unlikely since this cohort excluded such subjects.
We minimized Type I error by performing inferential analysis only to test pre-specified pharmacogenetic hypotheses on a single primary endpoint, and by doing only pairwise comparisons within different genotype groups when there was a significant interaction effect between treatment and genotype.
Predicated on our earlier report that early-onset alcoholics were more likely than late-onset alcoholics to respond therapeutically to ondansetron treatment (28
), we considered the possibility that age of onset could be a proxy for serotonin transporter genotype, or vice versa. Early-onset alcoholics differ from late-onset alcoholics by having an earlier age of alcohol-related problems, typically before the age of 25 years (although this can vary by definition), and increased rates of impulse-dyscontrol-related disorders (52
). Notably, in the present cohort, there was no significant association between serotonin transporter genotype and age of onset (data not shown), and those with the LL or LL/TT genotype responded to ondansetron irrespective of age of onset. Despite the lack of a significant association between age of onset and genotype, and contrary to expectation (53
), the slightly stronger trend was for those who had the LL genotype and were late-onset alcoholics to respond better to ondansetron than the LL early-onset alcoholics (data not shown). This also was unexpected because the preponderance of the literature suggests that it is the S allele that is more likely to be associated with an early onset of alcoholism (54
). Importantly, as exemplified by the contradictory reports in the literature, age of onset is difficult to standardize, especially across different populations and ethnicities, and there have been over 150 years of debate as to the constellation and stability of alcohol-related problems that should be used to make this characterization (52
). In contrast, the measurement of genetic variation is extremely accurate, and can be used reliably over time and across populations. A pharmacogenetic approach might, therefore, be a practical and useful method in general practice to identify those who should or should not receive ondansetron.
In sum, the results of our study have enabled us to integrate a unique scientific hypothesis, and a systematic series of basic research and human laboratory studies, with new clinical knowledge to demonstrate the promise of a novel pharmacogenetic approach to reduce the severity of alcohol consumption and increase abstinence among alcoholics with specific polymorphisms of the 5-HTT gene.