Nucleoside phosphonate prodrugs: synthesis and antiviral activity

2016-01-26Acyclic nucleoside phosphonates, ANPs, are a drug class with broad-spectrum activity against DNA and RNA viruses. However, limitations with ANP efficacy include: low cell membrane permeability, low oral bioavailability (due to the presence of an ionizable anionic group) and nephrotoxicity In an effort to curtail these effects, as well as to further understand and demonstrate the full therapeutic potential of this drug class, a main focus of the work presented in this dissertation is on the synthesis and antiviral activity of various ANP parent drugs and their corresponding acyclic and cyclic nucleoside phosphonate prodrugs. ❧ Parent ANPs were masked with an amino acid or dipeptide promoiety, which contained a lipophilic N-alkyl long chain on the C-terminal group as a carboxamide. PME and PMP ANP monoester prodrugs required an alternative synthetic route involving PyBrOP coupling with a small alkoxy nucleophile followed by PyBOP coupling with the desired amino acid or dipeptide promoiety. A novel (L)-Ser N-alkyl dipeptide cyclic prodrug of (S)-HPMPC was synthesized in order to evaluate and expand the ANP peptidomimetic prodrug library. PME and HPMP prodrugs containing an N-alkyl C₁₆ long chain demonstrated EC₅₀ values in the micromolar range against a variety of DNA viruses herpes simplex virus type 1, varicella zoster virus, cytomegalovirus, cowpox virus, adenovirus, and vaccinia virus. In addition, a selection of lipophilic N-alkyl amino acid- and dipeptide-based ANP prodrugs were evaluated for in vitro antiviral activity against Kaposi-sarcoma associated herpesvirus. (L)-Tyr-NHC₁₈H₃₇ and (L)-Ser-NHC₁₆H₃₃-(L)-Val prodrugs of (S)-HPMPA and (S)-HPMPC demonstrated inhibition values requiring further assessment of their application as KSHV antiviral agents. In addition, a large-scale synthesis (1 g – 2 g) of (L)-Tyr-NHC₁₆H₃₃ cyclic and acyclic HPMP prodrugs was performed as a means to assess reaction conditions for optimal and efficient generation of large quantities of lead candidates. ❧ Ribavirin (RBV) is a guanosine ribonucleic analogue that acts as a nucleoside inhibitor. It has been found to be active against various DNA and RNA viruses (including flaviviridae viruses like yellow fever and hepatitis C). In an effort to expand the druggable phosphonate library and potentially enhance its antiviral activity against dengue virus, an emerging RNA pathogen to humans, two 5’-phosphonate analogues of RBV have been synthesized. The structural modification lies in replacing the 5’-OH of the ribose ring with an X-P(O)(OH)₂ linkage (where X is CH₂ or CH₂-CH₂). The 5’-phosphonate RBV analogue was synthesized from a published procedure using the well-utilized Michaelis-Arbuzov reaction and the Horner-Wadsworth-Emmons reaction was employed to extend the alkyl length between the ribose ring and phosphonate to afford a novel 5’-methylene phosphonate RBV analogue