Synthesis and stability studies of phosphonoformate–amino acid conjugates: a new class of slowly releasing foscarnet prodrugs

Prodrugs of phosphonoformic acid (PFA), an anti-viral agent used clinically as the trisodium salt (foscarnet), are of interest due to the low bioavailability of the parent drug, which severely limits its utility. Neutral PFA triesters are known to be susceptible to P–C bond cleavage under hydrolytic de-esterification conditions, and it was previously found that P,C-dimethyl PFA P–N conjugates with amino acid ethyl esters did not release PFA at pH 7, and could not be fully deprotected under either acid or basic conditions, which led, respectively, to premature cleavage of the P–N linkage (with incomplete deprotection of the PFA ester moiety), or to P–C cleavage. Here we report that novel, fully deprotected PFA-amino acid P–N conjugates 4 can be prepared via coupling of C-methyl PFA dianion 2 with C-ethyl-protected amino acids using aqueous EDC, which gives a stable monoanionic intermediate 3 that resists P–C cleavage during subsequent alkaline deprotection of the two carboxylate ester groups. At 37 °C, the resulting new PFA-amino acid (Val, Leu, Phe) conjugates (4a–c) undergo P–N cleavage near neutral pH, cleanly releasing PFA. A kinetic investigation of 4a hydrolysis at pH values 6.7, 7.2, and 8.5 showed that PFA release was first-order in [4a] with respective t1/2 values of 1.4, 3.8, and 10.6 h.