Prodrug approaches to improving the oral absorption of antiviral nucleotide analogues

Abstract

Nucleotide analogues have been well accepted as therapeutic agents active against a number of viruses. However, their use as antiviral agents is limited by the need for phosphorylation by endogenous enzymes, and if the analogue is orally administered, by low bioavailability due to the presence of an ionizable diacid group. To circumvent these limitations, a number of prodrug approaches have been proposed. The ideal prodrug achieves delivery of a parent drug by attachment of a non-toxic moiety that is stable during transport and delivery, but is readily cleaved to release the parent drug once at the target. Here, a brief overview of several promising prodrug strategies currently under development is given.

Figure 1

General structures exemplifying the antiviral prodrug approaches discussed in this review.

Figure 2

A. General structures of pronucleotides investigated by McGuigan and coworkers. The alkyloxy phosphoramidates (1) were initially evaluated. Phosphorodiamidates (2) were also tested, but showed no biological advantage over the phosphoramidates. The diaryl triesters (3) lost antiviral activity in kinase-deficient cells. However, the aryloxy phosphoramidates (4) proved to be the most successful pronucleotides. B. Activation pathway of the aryloxy phosphoramidate pronucleotides. Step a involves cleavage of the ester group by an esterase to form intermediate 6. Hydrolysis of the aryl group facilitated by intramolecular cyclization of the amino acid moiety followed by hydrolysis produces the amino acyl metabolite (AAM, 7). Cleavage of the P-N bond by a phosphoramidase yields the 5′-nucleoside monophosphate 8 (step c). C. Structures of the amino acid phosphoramidate monoesters: tryptophan methyl ester (9), tryptophan methyl amide (10) and phenylalanine methyl ester (11). The L-Val, L-Leu and L-Ala methyl ester derivatives were also synthesized.

Figure 3

A. Activation of the SATE pronucleotides. The acyl group is cleaved to release the intermediate, which contains a reactive thiol group 13 (step a). Intramolecular cyclization to form an ethylene sulfide results in spontaneous decomposition to the diester or phosphoramidate 14 (step b). If the pronucleotide contains a second SATE moiety, the process is repeated until the 5′-nucleoside monophosphate is released. If the second group is an aryloxy group or a phosphoramidate, hydrolysis can either proceed sequentially or concomitantly to release the 5′-nucleoside monophosphate. B. General structures of the SATE pronucleotides: bis(SATE) triesters (15), bis(hydroxyl-tBuSATE) triesters (16), aryl SATE phosphotriesters (17), and SATE phosphoramidate diesters (18).

Figure 4

A. The triester pronucleotides studied by Meier and coworkers. The cycloSal pronucleotides (19) led to increased intracellular levels of NMP. The bis-cycloSal pronucleotides (20) are able to deliver two molecules of drug per biomolecule administered. B. Activation pathway for the cycloSal pronucleotides. Cleavage of the phenyl ester is initiated by nucleophilic attack at the phosphorus to produce the diester 22 (step a₁). Cleavage of the benzyl ester yields the nucleotide analogue 23 and salicyl alcohol 24 (step a₂). The benzyl ester of 21 can also first be hydrolyzed resulting in the charged intermediate 25 (step b₁), which then reacts with water to produce 26 (step b₂). Hydrolysis of 26 does not occur (step b₃), and the nucleotide is not released.

Figure 5

General structures of the ethylene glycol-linked amino acid (27) and serine phosphoester dipeptide pronucleotides (28 and 29).