Proposal for using small molecule tumor necrosis factor-alpha lowering agents, possibly bupropion, in aplastic anemia

Part Two: Evaluation of N-methylbupropion as a Potential Bupropion Prodrug

N-methylbupropion was selected as a potential prodrug from our in vitro screening of analogues of bupropion described in the preceding paper. This study describes in vivo pharmacokinetics of N-methylbupropion in the guinea-pig animal model, which is reported to best predict human metabolism of bupropion. The suitability of the guinea pig was established by studying N-demethylation of N-methylbupropion using S9 liver fractions. An LC-MS method was developed and validated to measure N-methylbupropion, bupropion and their metabolites in plasma and brain tissue. In separate studies, the prodrug was delivered by intraperitoneal injection (IP) to assess hepatic metabolism and then by oral gavage (PO) to assess the contribution from intestinal enzymes. Bupropion was administered in parallel. The pharmacokinetic profile of bupropion and N-methylbupropion were not comparable when dosed by intraperitoneal injection but when dosed orally, N-methylbupropion showed a comparable bupropion and metabolite PK plasma profile to bupropion. Plasma and brain levels of N-methylbupropion show that it is extensively metabolized to bupropion and its metabolites, and N-methyl-threo-hydrobupropion. This data coupled to the reduced DAT and NET system in vitro activity described in paper 1 would suggest that the N-methyl derivative of bupropion may have potential as an oral prodrug of bupropion in humans.

Synthesis, screening and pharmacokinetic evaluation of potential prodrugs of bupropion. Part one: in vitro development

In general, prodrugs are developed to circumvent deficiencies associated with the absorption, distribution, metabolism, excretion or toxicological (ADMET) profile associated with the active drug. In our study, we select bupropion, a drug with broad pharmacology incorporating dopaminergic, noradrenergic, nicotinic and cytokine modulation properties, but which is rapidly metabolized in vivo. We exploited its carbonyl and secondary amine functionality to facilitate the synthesis of bioprecursor prodrug forms with the sole objective of identifying analogues with enhanced properties over bupropion. A range of analogues were synthesized, ranging from N-methyl, N-benzyl, oximes, enol acetate and ether forms to examples where both functional groups were utilized to form oxadiazine, oxadiazinone, oxazolone and acetylated derivatives. We then developed an in vitro metabolic screen to simulate the human oral delivery route for these analogues. The selection of media in the screens contained a variety of pH, enzymatic and co-factor systems which mimic metabolic in vivo environments that drugs encounter when delivered orally. By coupling our in vitro screening tool to a selective hyphenated technique such as LC-MS, we were able to quickly select potential prodrugs for further in vitro and in vivo development. From the data generated, the N-alkylated bupropion analogues were shown to have the highest potential to act as bioprecursor prodrugs of bupropion.