Role of d-amphetamine and d-methamphetamine as active metabolites of benzphetamine: Evidence from drug discrimination and pharmacokinetic studies in male rhesus monkeys

- d-Amphetamine produced full cocaine-like stimulus effects in all monkeys.
- A clockwise hysteresis loop related d-amphetamine behavioral effects and levels.
- Benzphetamine produced full cocaine-like stimulus effects in 2 of 4 monkeys.
- Benzphetamine produced sequential d-methamphetamine and d-amphetamine plasma levels.
- Benzphetamine behavioral effects did not correlate with metabolite levels.

Benzphetamine is a Schedule III anorectic agent that is a prodrug for d-amphetamine and d-methamphetamine and may have utility as an “agonist” medication for cocaine use disorder treatment. This study evaluated the pharmacokinetic-pharmacodynamic profile of benzphetamine using a drug discrimination procedure in rhesus monkeys. The potency and time course of cocaine-like discriminative stimulus effects were compared for benzphetamine (10-18 mg/kg, intramuscular (IM)) and d-amphetamine (0.032-0.32 mg/kg, IM) in monkeys (n = 3-4) trained to discriminate IM cocaine (0.32 mg/kg) from saline in a two-key food-reinforced discrimination procedure. Parallel pharmacokinetic studies in the same monkeys determined plasma benzphetamine, d-methamphetamine and/or d-amphetamine levels for correlation with behavioral effects. d-Amphetamine produced dose-dependent, time-dependent, and full cocaine-like effects, i.e. ≥ 90% cocaine-appropriate responding, in all monkeys without altering response rates. The time course of d-amphetamine's cocaine-like discriminative stimulus effects correlated with plasma d-amphetamine levels. Benzphetamine was 180-fold less potent than d-amphetamine and produced full cocaine-like effects in only 2 of 4 monkeys while significantly decreasing response rates. Benzphetamine administration increased plasma d-methamphetamine (peak at 100 min) and d-amphetamine (peak at 24 h) levels, but the time course of behavioral effects did not correlate with increased levels of benzphetamine, d-methamphetamine or d-amphetamine. These results suggest that benzphetamine yields d-amphetamine and d-methamphetamine as active metabolites in rhesus monkeys, but generation of these metabolites is not sufficient to account for benzphetamine behavioral effects. The incomplete cocaine substitution profile and protracted d-amphetamine plasma levels suggest that benzphetamine may still warrant further evaluation as a candidate pharmacotherapy for cocaine use disorder treatment.