This study demonstrated that a cocaine vaccine that used a disrupted Ad gene transfer vector elicited a robust and long-lasting humoral immune response in rats. We recently reported, using the same vector coupled to the first generation of a cocaine analog hapten (GNC), that the vaccine elicited a high level of antibody titers over 3 months in mice (Hicks et al, 2011
). Therefore, disrupted adenovirus (dAd), a highly immunogenic cold virus, appears to be effective in producing strong immune responses against a non-immunogenic small-molecule drug in rodents. In sequestering drugs of abuse with antibodies, it is important to evoke both high titers and high-affinity antibodies to the drug to be able to counteract the bolus amount of a drug commonly used in addicts. In this study, the elicited antibodies increased the blood-to-brain ratio of cocaine distribution 7.5-fold, suggesting the functional specificity and efficacy of the antibodies to sequester free cocaine in the bloodstream sufficient to eliminate the subsequent cocaine-induced brain chemistry and subjective ‘high'. Therefore, we predicted that immunization with dAd5GNE would inhibit the effects of cocaine on addiction-related behaviors in rats.
When acutely injected with cocaine, dAd5GNE-vaccinated rats displayed a vastly reduced response to cocaine-induced stimulant behaviors in all measurable categories. Although the cocaine injection in both the control and vaccinated groups elicited hyperlocomotion and rearing, the dAd5GNE-vaccinated groups exhibited a greatly reduced magnitude of these behaviors (). In this study, the vaccinated rats showed a progressive decrease in the psychomotor-stimulating effect of cocaine during repeated injections of cocaine, whereas the non-vaccinated rats exhibited sensitization to the effect. Specifically, the hyperlocomotor activity that usually follows a high bolus dose of cocaine significantly decreased by 80% compared with naive control rats by the end of the experiment (ie, a 2.8-fold decrease; ). Similarly, rats typically display hyperactivity of vertical (rearing) events in response to cocaine, which was diminished in vaccinated rats to the near-normal levels of PBS control rats (). The effect remained throughout eight challenges of 15
mg/kg cocaine over a 3-week period, demonstrating the persistence of vaccine efficacy and a lack of antibody exhaustion caused by multiple cocaine exposures. In fact, we found a high level of antibodies when measured 1 week after the last boost, which corresponded to the time course of the development of blockade of the psychostimulant effects of cocaine in the vaccinated rats.
Vaccination did not alter the initial acquisition of cocaine self-administration in rats. However, vaccinated rats showed a downward shift of the cocaine dose–response function compared with control rats, similar to a previous observation with succinylnorcocaine attached to cholera B toxin (Kantak et al, 2000
). Most importantly, vaccinated rats exhibited decreased motivation to self-administer cocaine compared with control rats when the cost (ie, PR requirement) for a cocaine injection increased. This effect was specific to cocaine. Both the control and vaccinated groups showed no difference in their motivation to self-administer methamphetamine and food. These PR results show a profile similar to lowering the dose of cocaine (Richardson and Roberts, 1996
) and strongly suggest that vaccination decreases the reinforcing effects of cocaine. In humans, subjects with high cocaine antibody titers also reported significantly reduced ‘good' drug effect and cocaine quality after smoking cocaine (Haney et al, 2010
The finding that the vaccinated and control rats showed the same response to acute cocaine in the initial locomotor response to cocaine and no significant difference in the initial acquisition of cocaine self-administration deserves comment. One possible explanation for the locomotor activity may be the incomplete development of the control response to the drug combined with high spontaneous activity at these initial time-points. The locomotor response in the control animals is not particularly robust upon the initial injection of this modest dose of cocaine and only becomes substantial with repeated administration. We hypothesize that the initial hyperlocomotive response to cocaine may be less sensitive to blockade of cocaine by an antibody response as this behavior was less robust and thus less responsive to changes in cocaine dose. Similarly, acquisition of cocaine self-administration can occur over a wide range of doses, suggesting that this measure is also less sensitive to changes in dose. In contrast, sensitization of hyperlocomotion, PR performance, and drug-induced reinstatement of responding are particularly sensitive to a dose change in cocaine.
Consistent with the robust effects of the vaccine on motivation to seek cocaine in this study, we did not observe a ‘compensatory-like' increase in cocaine self-administration in vaccinated rats as has been observed with low doses of dopamine antagonists and low antibody titers from other studies (Caine and Koob, 1994
; Kantak et al, 2001
). For example, previous results showed that passive immunization with a low concentration of monoclonal antibodies produced an upward shift of the cocaine dose–response function in rats, and a high concentration of monoclonal antibodies produced a downward shift as observed in this study (Kantak et al, 2001
). Note again that all self-administration testing in this study was performed after a complete immunization. Therefore, high stable antibody titers at the time of exposure to cocaine self-administration may have caused a downward shift, rather than a compensatory increase, in cocaine self-administration by vaccinated rats.
A major challenge in treating drug addiction is to prevent relapse to drug use after prolonged drug abstinence. The literature indicates that stress, drug-associated cues, and brief exposure to the drug itself can trigger drug craving and relapse to drug use in humans (Epstein et al, 2006
; Yahyavi-Firouz-Abadi and See, 2009
). Similarly, it is well established that animals that undergo extinction of drug self-administration reinstate responding when they are primed with a previously self-administered drug or other drugs of the same class (Stewart, 2000
). This phenomenon is termed drug-primed reinstatement in animals. When saline was initially substituted for cocaine in this study, the control rats showed ‘extinction burst' responding, which is commonly observed in actively learned responses (Harris et al, 2007
), whereas the vaccinated rats did not. This observation is very similar to observations of ‘extinction burst' responding in rats with high cocaine intake and no ‘extinction bursts' in rats with low cocaine intake (Edwards et al, 2007
). In addition, when primed with cocaine, control rats reinstated responding on the previously cocaine-associated lever, whereas vaccinated rats did not, consistent with a previous finding (Carrera et al, 2000
). Consequently, the data suggest that the antibodies prevented primed cocaine from reaching the brain to initiate cocaine-seeking mechanisms in rats.
In summary, this study demonstrated that a disrupted Ad vector-based vaccine coupled to a third-generation cocaine hapten GNE was able to induce antibody titers at a sufficiently high level to antagonize the psychomotor-stimulating and reinforcing effects of cocaine in rats. In addition, immunization with dAd5GNE inhibited cocaine seeking in rats when briefly exposed to cocaine after the extinction of cocaine self-administration. This study lasted approximately 4 months, supporting the long-lasting effect of dAd5GNE immunization in rats. Therefore, these data strongly suggest that a disrupted Ad gene therapy vector-based vaccine may be effective in the treatment of cocaine abuse and addiction in humans.