Alkoxyalkyl prodrugs of acyclic nucleoside phosphonates enhance oral antiviral activity and reduce toxicity: current state of the art

Abstract

Although the acyclic nucleoside phosphonates cidofovir, adefovir and tenofovir are approved for treating human cytomegalovirus, hepatitis B and HIV infections, respectively, their utility is limited by low oral bioavailability, renal toxicity and poor cell penetration. Research over the past decade has shown that these undesirable features can be eliminated by esterifying the compounds with an alkoxyalkyl group, in effect disguising them as lysophospholipids. In this modified form, the drugs are readily taken up in the gastrointestinal tract and have a prolonged circulation time in plasma. The active metabolite also has a long half life within cells, permitting infrequent dosing. Because these modified drugs are not recognized by the transport mechanisms that cause the accumulation of acyclic nucleoside phosphonates in renal tubular cells, they lack nephrotoxicity. Alkoxyalkyl esterification also markedly increases the in vitro antiviral activity of acyclic nucleoside phosphonates by improving their delivery into cells. For example, an alkoxyalkyl ester of cyclic-cidofovir, a less soluble compound, retains anti-CMV activity for 3 months following a single intravitreal injection. Two of these novel compounds, hexadecyloxypropyl-cidofovir (CMX001) and hexadecyloxypropyl-tenofovir (CMX157) are now in clinical development. This article focuses on the hexadecyloxypropyl and octadecyloxyethyl esters of cidofovir and (S)-HPMPA, describing their synthesis and the evaluation of their in vitro and in vivo activity against a range of orthopoxviruses, herpesviruses, adenoviruses and other double-stranded DNA viruses. The extension to other nucleoside phosphonate antivirals is highlighted, demonstrating that this novel approach can markedly improve the medicinal properties of these drugs.

Figure 1

Acyclic Nucleoside Phosphonate Antivirals in Clinical Use

Figure 2

Interactions of phosphatidylcholine, lysophosphatidylcholine and HDP-CDV with phospholipid bilayer membranes

Figure 3

Predicted major metabolic cleavage sites for lysophosphatidylcholine and HDP-CDV

Figure 4

Structure of Key Alkoxyalkyl Esters of CDV and (S)-HPMPA

Figure 5

Effect of alkoxypropyl-cidofovir chain length on antiviral activity against HCMV, MCMV and drug-resistant HCMV isolates Left Panel: Wild type CMVs; HCMV (AD169), circles; MCMV, squares; Right panel: Drug resistant CMV isolates: VR4760 (GCV/PFA resistant, UL97/DNA pol mutations), triangles; C8914-6 (GCV resistant, UL 97 mutation), circles; 759 D100 (GCV/CDV resistant, DNA pol mutation), squares. Adapted from Wan et al, 2005 and Williams-Aziz et al, 2005

Figure 6

Cellular uptake of CDV, (S)-HPMPA and their HDP esters in MRC-5 cells Left panel: CDV, circles; HDP-CDV, squares; Right panel: (S)-HPMPA, circles; HDP-(S)-HPMPA, squares. Adapted from Aldern et al, 2003, and Magee et al, 2008.

Figure 7

Metabolism and anabolic phosphorylation of CDV, (S)-HPMPA and their HDP esters in MRC-5 cells in vitro Legend: 10 μM ¹⁴C-labeled CDV, (S)-HPMPA, HDP-CDV and HDP-(S)-HPMPA were applied to wells containing confluent MRC-5 cells and incubated for 24 hrs. Anabolic phosphorylation to their monophosphates (CDVp and (S)-HPMPAp) and diphosphates (CDVpp and (S)-HPMPApp) was measured by Partisil SAX HPLC. Adapted from Aldern et al, 2003, and Magee et al, 2008.

Figure 8

Key pharmacokinetic features of [2-¹⁴C]-CDV and HDP-[2-¹⁴C]-CDV after administration to mice: plasma and kidney levels of drug and metabolites. Legend. Upper panel: Plasma levels of [¹⁴C]-CDV (triangles) or HDP-[¹⁴C]-CDV (circles) after oral administration to mice at 17.8 μmoles/kg. Lower panel: Kidney levels of [2-¹⁴C]-CDV (squares) after intraperitoneal administration (17.8 μmoles/kg) or HDP-[2-¹⁴C]-CDV (circles) after oral administration (17.8 μmoles/kg) to mice. Adapted from Ciesla et al, 2003.

Figure 9

Proposed pathways of intestinal absorption of HDP-CDV Legend: HDP-CDV (dark symbols) in mixed micelles of bile salts, monoglycerides, and lysophospholipids is absorbed at the brush border of the enterocyte. HDP-CDV enters the portal vein directly. Chylomicrons are formed in the endoplasmic reticulum with a core consisting of triglycerides (TG) and cholesterol esters and a surface coat consisting of apoprotein B-48 (not shown in the diagram), phosphatidylcholine (light symbols) and HDP-CDV (dark symbols) and secreted into the small intestinal lymphatics.

Figure 10

Structure of alkoxyallkyl esters of various antiviral acyclic nucleoside phosphonates Abbreviations: B, nucleobase; R, alkoxyalkyl; HPMP, 3-hydroxy-2-(phosphono-methoxy)propyl; PME, 2-(phosphonomethoxy)ethyl; PPen, 5-phosphono-pent-2-en-1-yl; HPPMP, 3-hydroxy-2-(phosphophosphomethoxy)propyl; PMP, 3-hydroxy-2-(phosphono-methoxy)propyl; PPM, phosphonopropoxymethyl.