Dideoxynucleosides, which are potent inhibitors of HIV reverse transcriptase and other viral DNA polymerases, are a common component of highly active anti-retroviral therapy (HAART) (ref. 1). Six reverse transcriptase inhibitors have been approved for human use: azidothymidine; 2'3'-dideoxycytidine; 2'3'-dideoxyinosine; 2', 3'-didehydro-3'deoxythymidine; 2',3'-dideoxy-3'-thiacytidine; and 4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]-2-cyclopentene-1-++ +metha nol. Although drug-resistant HIV strains resulting from genetic mutation have emerged in patients treated with HAART (ref. 1), some patients show signs of drug resistance in the absence of drug-resistant viruses. In our study of alternative or additional mechanisms of resistance operating during antiviral therapy, overexpression and amplification of the MRP4 gene correlated with ATP-dependent efflux of PMEA (9-(2-phosphonylmethoxyethyl)adenine) and azidothymidine monophosphate from cells and, thus, with resistance to these drugs. Overexpression of MRP4 mRNA and MRP4 protein severely impaired the antiviral efficacy of PMEA, azidothymidine and other nucleoside analogs. Increased resistance to PMEA and amplification of the MRP4 gene correlated with enhanced drug efflux; transfer of chromosome 13 containing the amplified MRP4 gene conferred resistance to PMEA. MRP4 is the first transporter, to our knowledge, directly linked to the efflux of nucleoside monophosphate analogs from mammalian cells.

Bis(isopropyloxymethylcarbonyl) 9-R-(2-phosphonomethoxypropyl)adenine [bis(POC)PMPA] has been identified as a novel prodrug of PMPA. The anti-human immunodeficiency virus activity of bis(POC)PMPA was >100-fold greater than that of PMPA in both an established T-cell line and primary peripheral blood lymphocytes. This improved efficacy was shown to be due to a rapid intracellular uptake of the prodrug resulting in an increased intracellular accumulation of PMPA diphosphate (PMPApp), the pharmacologically active metabolite. PMPApp levels in bis(POC)PMPA-treated cells exceeded by >1,000-fold the levels seen in cells treated with unmodified PMPA in both resting and activated peripheral blood lymphocytes. Significant differences in the intracellular catabolism of PMPA metabolites were noted between the resting and activated lymphocytes. The half-life for the disappearance of PMPApp, derived from either bis(POC)PMPA or PMPA, was 12 to 15 h in the activated lymphocytes and 33 to 50 h in the resting lymphocytes. This long persistence of PMPApp, particularly in resting lymphocytes, may be unique to the nucleoside phosphonate analogs and indicates that effective levels of the active metabolite can be achieved and maintained with relatively infrequent administration of the parent drug.

The envelope glycoproteins of Rous sarcoma virus (RSV), gp85 and gp37, are anchored in the membrane by a 27-amino acid, hydrophobic domain that lies adjacent toa 22-amino acid, cytoplasmic domain at the carboxy terminus of gp37. We have altered these cytoplasmic and transmembrane domains by introducing deletion mutations into the molecularly cloned sequences of a proviral env gene. The effects of the mutations on the transport and subcellular localization of the Rous sarcoma virus glycoproteins were examined in monkey (CV-1) cells using an SV40 expression vector. We found, on the one hand, that replacement of the nonconserved region of the cytoplasmic domain with a longer, unrelated sequence of amino acids (mutant C1) did not alter the rate of transport to the Golgi apparatus nor the appearance of the glycoprotein on the cell surface. Larger deletions, extending into the conserved region of the cytoplasmic domain (mutant C2), resulted in a slower rate of transport to the Golgi apparatus, but did not prevent transport to the cell surface. On the other hand, removal of the entire cytoplasmic and transmembrane domains (mutant C3) did block transport and therefore did not result in secretion of the truncated protein. Our results demonstrate that the C3 polypeptide was not transported to the Golgi apparatus, although it apparently remained in a soluble, nonanchored form in the lumen of the rough endoplasmic reticulum; therefore, it appears that this mutant protein lacks a functional sorting signal. Surprisingly, subcellular localization by internal immunofluorescence revealed that the C3 protein (unlike the wild type) did not accumulate on the nuclear membrane but rather in vesicles distributed throughout the cytoplasm. This observation suggests that the wild-type glycoproteins (and perhaps other membrane-bound or secreted proteins) are specifically transported to the nuclear membrane after their biosynthesis elsewhere in the rough endoplasmic reticulum.The mechanism(s) by which cells send membrane-bound and secreted proteins to their proper subcellular locations remains an enigma of molecular biology. The mechanism presumably involves the specific interaction of "sorting signals," located within the structure of the newly synthesized proteins, with membrane-bound receptors in the rough endoplasmic reticulum (RER)' and Golgi apparatus of the cell (for review, see references 53 and 59). Evidence that protein sorting requires such specific interactions has recently been provided by the Abbreviations used in this paper. Endo H, endoglycosidase H" HA, hemagglutinin; RER, rough endoplasmic reticulum; RSV, Rous sarcoma virus; VSV, vesicular stomatitis virus. observation that cells can transport and secrete a variety of glycosylated and unglycosylated proteins at distinctly different rates (10,18, 36, 38,61).Very little is known about the composition(s) of sorting signals, but it is generally thought that they are composed of protein. In support of this idea, transport defective mutants of secreted proteins have be...

The human immunodeficiency virus type 1 (HIV-1) protease inhibitors (PIs)—saquinavir, ritonavir, nelfinavir, and indinavir—interact with the ABC-type multidrug transporter proteins MDR1 and MRP1 in CEM T-lymphocytic cell lines. Calcein fluorescence was significantly enhanced in MDR1+ CEM/VBL100 and MRP1+CEM/VM-1-5 cells incubated in the presence of various HIV PIs and calcein acetoxymethyl ester. HIV PIs also enhanced the cytotoxic activity of doxorubicin, a known substrate for MDR1 and MRP1, in both VBL100 and VM-1-5 CEM lines. Saquinavir, ritonavir, and nelfinavir enhanced doxorubicin toxicity in CEM/VBL100 cells by approximately three- to sevenfold. Saquinavir and ritonavir also enhanced doxorubicin toxicity in CEM/VM-1-5 cells. HIV-1 replication was effectively inhibited by the various PIs in all of the cell lines, and the 90% inhibitory concentration for a given compound was comparable between the different cell types. Therefore, overexpression of MDR1 or MRP1 by T lymphocytes is not likely to limit the antiviral efficacy of HIV PI therapy.

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Bis(pivaloyloxymethyl) [bis(pom)J derivatives of various acyclic nucleoside phosphonates-9-(2-phosphonylmethoxyethyl)adenine (PMEA), 9-(2-phosphonylmethoxypropyl)adenine (PMPA), and 9-(2-phosphonylmethoxypropyl)diaminopurine (PMPDAP)-were found to exhibit 9-to 23-fold greater antiviral activity than their corresponding unmodified compounds. The cytotoxicity of the bis(pom) analogs was also increased by various degrees, thus altering the therapeutic indexes of these compounds. Metabolic studies using [3H]bis-(pom)PMEA and [3H]PMEA as model compounds suggested a > 100-fold increase in the cellular uptake of the bis(pom) derivative and formation of active diphosphorylated metabolite. However, the bis(pom) derivatives were chemically unstable and highly susceptible to serum-mediated hydrolysis, factors which limit their potential utility for intracellular drug delivery.Acyclic nucleoside phosphonate analogs of both purine and pyrimidine bases show a broad-spectrum antiviral activity against several RNA and DNA viruses (reviewed in reference 4). The adenine analog, 9-(2-phosphonylmethoxyethyl)adenine (PMEA), is active against human immunodeficiency virus type 1 (HIV-1) and other retroviruses, including HIV-2, simian immunodeficiency virus, feline immunodeficiency virus, and Moloney murine sarcoma virus. PMEA is also active against various herpesviruses, including herpes simplex virus type 1 (HSV-1), HSV-2, cytomegalovirus, and Epstein-Barr virus. Thus, PMEA is of interest both as a potential antiretroviral drug for HIV-1 infections and also for the treatment of some of the opportunistic infections associated with AIDS, and it is currently undergoing a phase I and II trial for the evaluation of its toxicity and/or efficacy in AIDS patients. The related phosphonate analogs 9-(2-phosphonylmethoxypropyl)adenine (PMPA) and 9-(2-phosphonylmethoxypropyl)diaminopurine (PMPDAP) exhibit potent anti-HIV activity, although these compounds are less effective against herpesviruses. The phosphonyl groups exhibit a negative charge at the physiological pH, and hence the cellular uptake and bioavailability (in rats) of these molecules with oral drug administration are relatively poor (8). Recently, bis(pivaloyloxymethyl) [bis-(pom)] esters of the antitumor nucleotide analogs 5-fluoro-2'-deoxyuridine were shown to function as membrane-permeable prodrugs and inhibit proliferation of both wild-type and thymidine kinase-deficient murine leukemia cells (5, 9). Similarly, a bis(pom) ester of PMEA was found to have increased anti-herpes simplex virus activity in vitro compared with PMEA (11). However, the mechanisms involved in the increased activity of the bis(pom) derivative were not determined. In addition, little is known about the stability of the bis(pom) derivatives or their bioconversion to the active * Corresponding author. intracellular metabolites. In this study, we have evaluated the anti-HIV activity of bis(pom) esters of PMEA, PMPA, and PMPDAP and their metabolism in human lymphoid cell lines. Table 1 summarizes the anti-HIV ...

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In this report, we describe a new method to measure intracellular zidovudine triphosphate (ZDV-TP) levels in peripheral blood mononuclear cells (PBMCs) from patients treated with ZDV by utilizing inhibition of human immunodeficiency virus type 1 reverse transcriptase activity by ZDV-TP. Intracellular levels of ZDV-TP were determined with our enzymatic assay in PBMCs isolated from the blood of healthy individuals incubated with different concentrations of labeled ZDV and were validated by high-performance liquid chromatography separation and liquid scintillation counting of the radioactive ZDV-TP. These methods gave virtually identical results over a range of ZDV-TP concentrations from 150 to 900 fmol. ZDV-TP recoveries were over 90%o, and the limit of quantitation of ZDV-TP by this method was 20 to 50 fmol. To demonstrate the utility of the method, plasma ZDV and intracellular ZDV-TP concentrations were measured at serial time points over 6 h in 12 human immunodeficiency virus-infected volunteers following a single 100-or 500-mg oral dose of ZDV. Systemic oral clearance rates were similar to those in previous studies with adults but were highly variable (range, 0.86 to 2.75 liters/h/kg of body weight). The area under the plasma concentration versus time curve increased significantly (P < 0.0005) with the dose from a median value of 1.2 mg. h/liter at the lower dose to 4.2 mg. h/liter at the higher dose. Median intracellular ZDV-TP levels ranged from 5 to 57 and 42 to 92 fmol/106 cells in volunteers administered 100 and 500 mg of ZDV, respectively. Intracellular ZDV-TP levels rose to a plateau value by 2 h and remained consistent to 6 h. Although the higher dose and higher areas under the curve yielded consistently higher intracellular ZDV-TP levels, systemic pharmacokinetics explains only a modest proportion of the variability in cellular pharmacokinetics. The ZDV-TP bioassay should prove useful in further studies of ZDV metabolism in patient-derived PBMCs at the doses of ZDV currently administered.Zidovudine (3'-azido-3'-deoxythymidine [ZDV]) is one of three nucleoside analogs approved for the treatment of AIDS and AIDS-related complex (3,5,6,10,11,13,16). Investigation of the mode of action of ZDV has shown that the drug is phosphorylated to its 5'-monophosphate (MP), diphosphate, and triphosphate (TP) derivatives by cellular kinases. ZDV-5'-TP (ZDV-TP hereafter) is a potent inhibitor of human immunodeficiency virus reverse transcriptase (HIV-RT) and thus of HIV replication (8,12). Much of what is presently known about the intracellular metabolism of ZDV has been elucidated with radiolabeled drugs in cultured human lymphoid cells. For example, marked differences in the activation and accumulation of ZDV nucleotides have been noted among different cells upon incubation with ZDV which correlate with differences in the in vitro effectiveness of the drug (1, 2).Because in vitro results from human cell lines in culture cannot necessarily be extrapolated to the in vivo situation and because differences in drug dispo...