Selection and characterisation of murine leukaemia L1210 cells with high-level resistance to the cytostatic activity of the acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl) adenine (PMEA)

Interactions of 1-[(S)-3-Hydroxy-2-(phosphonomethoxy)propyl]cytosine (Cidofovir) Diphosphate with DNA Polymerases α, δ and ε*

The inhibitory and/or substrate activity of 1-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]cytosine [(S)-HPMPC, cidofovir, Vistide™] diphosphate towards eukaryotic DNA polymerases α, δ and ε* was examined. Cidofovir diphosphate is a weak competitive inhibitor of the above enzymes, approximately 3 to 7 times weaker than its adenine analogue (S)-HPMPApp. The enzymes also catalyze incorporation of (S)-HPMPC into DNA; after insertion of one (S)-HPMPC residue into DNA, another dNMP residue may incorporate. DNA polymerase δ and ε* can successively accommodate in the growing chain two (S)-HPMPC residues at the maximum, whereas pol α up to three residues.

Carrier-mediated delivery of 9-(2-phosphonylmethoxyethyl)adenine to parenchymal liver cells: a novel therapeutic approach for hepatitis B

Our aim is to selectively deliver 9-(2-phosphonylmethoxyethyl)adenine (PMEA) to parenchymal liver cells, the primary site of hepatitis B virus (HBV) infection. Selective delivery is necessary because PMEA, which is effective against HBV in vitro, is hardly taken up by the liver in vivo. Lactosylated reconstituted high-density lipoprotein (LacNeoHDL), a lipid particle that is specifically internalized by parenchymal liver cells via the asialoglycoprotein receptor, was used as the carrier. PMEA could be incorporated into the lipid moiety of LacNeoHDL by attaching, via an acid-labile bond, lithocholic acid-3alpha-oleate to the drug. The uptake of the lipophilic prodrug (PMEA-LO) by the liver was substantially increased after incorporation into LacNeoHDL. Thirty minutes after injection of [(3)H]PMEA-LO-loaded LacNeoHDL into rats, the liver contained 68.9% +/- 7.7% of the dose (free [(3)H]PMEA, <5%). Concomitantly, the uptake by the kidney was reduced to <2% of the dose (free [(3)H]PMEA, >45%). The hepatic uptake of PMEA-LO-loaded LacNeoHDL occurred mainly by parenchymal cells (88.5% +/- 8.2% of the hepatic uptake). Moreover, asialofetuin inhibited the liver association by >75%, indicating uptake via the asialoglycoprotein receptor. The acid-labile linkage in PMEA-LO, designed to release PMEA during lysosomal processing of the prodrug-loaded carrier, was stable at physiological pH but was hydrolyzed at lysosomal pH (half-life, 60 to 70 min). Finally, subcellular fractionation indicates that the released PMEA is translocated to the cytosol, where it is converted into its active diphosphorylated metabolite. In conclusion, lipophilic modification and incorporation of PMEA into LacNeoHDL improves the biological fate of the drug and may lead to an enhanced therapeutic efficacy against chronic hepatitis B.

N6-cyclopropyl-PMEDAP: a novel derivative of 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP) with distinct metabolic, antiproliferative, and differentiation-inducing properties

N6-Cyclopropyl-PMEDAP (cPr-PMEDAP) is a novel derivative of the acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP). Its cytostatic activity was found to be 8- to 20-fold more pronounced than that of PMEDAP and equivalent to that of the guanine derivative 9-(2-phosphonylmethoxyethyl)guanine (PMEG) against a variety of tumor cell lines. Unlike PMEDAP, but like PMEG, cPr-PMEDAP was equally cytostatic to wild-type and 9-(2-phosphonylmethoxyethyl)adenine/PMEDAP-resistant variants of the human erythroleukemia K562 and the murine leukemia L1210 cell lines. Also, cPr-PMEDAP and PMEG proved to be equipotent inducers of K562 and rat choriocarcinoma RCHO cell differentiation, whereas the differentiation-inducing activity of PMEDAP was 5- to 25-fold less pronounced. Furthermore, compared to PMEDAP, cPr-PMEDAP and PMEG were 10- to 25-fold more potent in inhibiting the progression of K562 cells through the S phase of the cell cycle, resulting in a marked accumulation of the four 2'-deoxyribonucleoside 5'-triphosphate pools. The biological effects of cPr-PMEDAP, but not PMEDAP, were reversed by the adenylate deaminase inhibitor 2'-deoxycoformycin (dCF). Formation of the deaminated derivative of cPr-PMEDAP (i.e. PMEG) was demonstrated in crude extracts from K562 and L1210 cells and in metabolism studies with radiolabeled cPr-PMEDAP and PMEG. This is the very first example of an acyclic nucleoside phosphonate analogue that is susceptible to deamination. However, cPr-PMEDAP was not recognized as a substrate by purified adenosine deaminase or by adenylate deaminase. These findings might point to an as yet unidentified cellular enzyme, sensitive to dCF but different from the common adenosine and AMP deaminases. Our data demonstrate the superior antiproliferative and differentiation-inducing effects of cPr-PMEDAP on tumor cells, as compared to the parent compound PMEDAP, based on the unique metabolic properties of this novel compound.

Enhanced 9-(2-phosphonylmethoxyethyl)adenine secretion by a specific, indomethacin-sensitive efflux pump in a mutant 9-(2-phosphonylmethoxyethyl)adenine-resistant human erythroleukemia K562 cell line

We have investigated the molecular basis of the 100-fold resistance of mutant human erythroleukemia K562/PMEA-1 cells to the antiproliferative potential of 9-(2-phosphonylmethoxyethyl)adenine (PMEA). Upon exposure to high PMEA concentrations, comparable intracellular PMEA levels were initially observed in mutant K562/PMEA-1 and wild-type K562/0 cells, indicating that PMEA influx was unaltered. However, after 4 hr of exposure to 0.2 microM [3H]bis(pivaloyloxymethyl)-PMEA [bis(POM)-PMEA], the total intracellular level of unphosphorylated and mono- and diphosphorylated PMEA was 2.8-fold lower in K562/PMEA-1 than in K562/0 cells. Increased PMEA secretion from K562/PMEA-1 cells (compared with K562/0 cells) became more pronounced upon prolonged exposure to bis(POM)-PMEA; after 24 hr, K562/PMEA-1 cells showed 65-fold lower total intracellular PMEA levels than K562/0 cells and at 48 hr, >400-fold less total PMEA was detected in K562/PMEA-1 cells. In addition, PMEA phosphorylation was 25- to 50-fold less efficient in K562/PMEA-1 than in K562/0 cells, pointing to an additional defect at the level of the metabolism of PMEA. The PMEA efflux mechanism was shown to be temperature- and azide-dependent, was markedly inhibited by indomethacin, and did not recognize adenine nucleotides or the phosphorylated metabolites of 3'-azido-3'-deoxythymidine. Also, over a 28-hr period, PMEA efflux was not affected by an inhibitor of RNA synthesis (actinomycin D) or protein synthesis (cycloheximide). Our studies revealed that resistance of K562/PMEA-1 cells to PMEA is the combined result of a severely impaired PMEA phosphorylation on the one hand, and an enhanced PMEA secretion by a highly specific, indomethacin-sensitive efflux pump, different from the classical P-glycoprotein- and multidrug resistance protein-mediated resistance mechanisms, on the other hand.