Enzymatic barrier to oral and central nervous system delivery of anti-HIV nucleoside reverse transcriptase inhibitors

Most of the dideoxynucleoside reverse transcriptase inhibitors used for treating HIV infection in humans exhibit very low central nervous system (CNS) uptake due to unknown reason(s). A clear understanding of the factors governing the CNS uptake of these dideoxynucleosides and new strategies to enhance their CNS uptake are needed to fully eradicate HIV infection and to treat AIDS dementia complex in patients with AIDS. An initial aim of this thesis is to examine the relative contributions of metabolism by purine salvage pathway enzymes localized in the CNS, namely, purine nucleoside phosphorylase (PNP) and adenosine deaminase (ADA), to the low CNS uptake of dideoxynucleosides which are substrates for these enzymes. Exploration of brain uptake using microdialysis was shown to be unreliable for studying transport of relatively hydrophillic molecules due to damage to the BBB during microdialysis probe insertion. Therefore, classical methods were employed in all brain uptake experiments. Further studies demonstrated that PNP does not play an important role in the CNS uptake of 2',3'-dideoxyinosine (ddI), probably due to its low affinity for ddI. On the other hand, 2sp/prime-fluoro-2',3'-dideoxyadenosine (FddA), a new anti-HN agent, is metabolized by brain tissue localized ADA resulting in the low CNS concentration of this compound. Inhibition of ADA present in the brain tissues significantly increases the CNS levels of F-ddA. Although ADA localized in the brain tissue prevents the entry of F-ddA into the CNS, its presence has been used productively to enhance the CNS uptake of other dideoxynucleosides. 6-chloro-2',3'-dideoxypurine (6-Cl-ddP), an ADA activated prodrug of ddI, provides 10-fold higher CNS concentrations of ddI compared to ddI controls after intravenous infusions. Unfortunately, the oral bioavailability of 6-Cl-ddP was determined in this study to be only 7% in rats due to its presystemic bioconversion to ddI by ADA present in the intestinal wall. A final aim of this thesis was to design approaches to enhance the intestinal absorption of ADA activated prodrugs without affecting their systemic or brain tissue bioconversion. Using in situ intestinal perfusions in rats with mesenteric cannulation combined with in vivo pharmacokinetic studies of a potent inhibitor of ADA, erythro-9-(2-hydroxy-3-nonyl)-adenine, a strategy was devised which may prove effective in inhibiting almost all intestinal ADA with only minimal inhibition of systemic ADA. Hence, the local inhibition approach may be used to enhance the oral delivery of brain-tissue activated prodrugs of anti-HIV agents.