Effects of 2',3'-dideoxynucleosides on mammalian cells and viruses

Polymerases of paramyxoviruses and pneumoviruses

The paramyxo- and pneumoviruses are members of the order Mononegavirales, a group of viruses with non-segmented, negative strand RNA genomes. The polymerases of these viruses are multi-functional complexes, capable of transcribing subgenomic capped and polyadenylated mRNAs and replicating the genome. Although there is no native structure available for any complete paramyxo- or pneumovirus polymerase, functional and structural studies of a fragment of a pneumovirus polymerase protein and mutation analyses and resistance profiling of small-molecule inhibitors have generated a wealth of mechanistic information. This review integrates these data with the structure of a related polymerase, identifying similarities, differences, gaps in knowledge, and avenues for antiviral drug development.

Synthesis of 2′,3′,5′-trideoxyuridine-5′-methylphosphonic Acid and its Diphosphate Derivative. Study of the Interaction with HIV-1 reverse Transcriptase

2′,3′,5′-Trideoxyuridine-5′-methylphosphonate, [8], was synthesized from ddU. The 5′,6′ carbon-carbon bond was formed by condensing the 5′-aldehyde of ddU and a Wittig reagent. The binding interaction of the diphosphate derivative [10] of the phosphonate [8] with HIV-1 reverse transcriptase (RT) was studied using methods based primarily on fluorescence spectroscopy. From the quenching of intrinsic tryptophan fluorescence of RT during its titration against [10], a dissociation constant of 24 μm was calculated at 25°C. In the presence of a DNA/DNA template/primer of defined sequence and RT, [10] and a fluorescent derivative of ddTTP competed for binding to RT without incorporation of ddU-5′-methylphosphonate. In the presence of a 0.5 mm concentration of [10], the RT activity measured with Poly(rA)/(dT)15 and [3H] dTTP was lowered to 65%. All our observations are consistent with suppression of the catalysis of bond formation between the OH at the primer 3′-end and α-P of [10] after formation of the complex between RT, template/primer and [10].

Didanosine

The evaluation and development of didanosine as a therapy for HIV-infection has demonstrated that results of cell-culture studies are only approximate predictors of clinical effects. Whereas the antiviral activity of didanosine is achieved in cell culture at concentrations much higher than those of zidovudine, the two drugs appear to achieve similar effects on markers of HIV-infection at similar clinical doses. The effect of the differing intracellular-half-life of the respective nucleoside triphosphates on the clinical effects is unknown. Similarly, preclinical toxicology studies, while suggesting a low potential for didanosine-induced haematological toxicities, did not predict the findings of peripheral neuropathy and pancreatitis in clinical studies. Thus, the results of controlled clinical studies are required in order to more fully define the therapeutic and safety profile of didanosine.

Zalcitabine (ddC) Phosphorylation and Drug Interactions

Zalcitabine (2′,3′-dideoxycytidine; ddC) is an inhibitor of HIV reverse transcriptase. The intracellular metabolism of ddC in peripheral blood mononuclear cells (PBMCs), U937 cells and Molt 4 cells were investigated, and phosphate metabolites were determined by on-line radiometric HPLC. Comparable levels of all three ddC phosphate metabolites were formed in PHA-stimulated PBMCs, U937 cells and Molt 4 cells.

Zidovudine (ZDV), didanosine (ddl) and stavudine (d4T) had no significant effect on ddC (0.06μM) phosphorylation in PBMCs whereas the endogenous nucleoside, cytidine decreased phosphorylation in a concentration-dependent manner ( e.g. 41 % inhibition of total phosphate formation at 6μM cytidine, 85% inhibition at 60μM).

The cytotoxic anticancer drug doxorubicin caused a decrease in ddC phosphorylation in U937/Molt 4 cells (e.g. 56% inhibition of total phosphate formation in U937 cells; 55% in Molt 4 cells at a doxorubicin concentration of 60μM), whilst the antiviral agent ribavirin exhibited no inhibitory effects.

Regulation of DNA replication at the end of the mitochondrial D-loop involves the helicase TWINKLE and a conserved sequence element

The majority of mitochondrial DNA replication events are terminated prematurely. The nascent DNA remains stably associated with the template, forming a triple-stranded displacement loop (D-loop) structure. However, the function of the D-loop region of the mitochondrial genome remains poorly understood. Using a comparative genomics approach we here identify two closely related 15 nt sequence motifs of the D-loop, strongly conserved among vertebrates. One motif is at the D-loop 5'-end and is part of the conserved sequence block 1 (CSB1). The other motif, here denoted coreTAS, is at the D-loop 3'-end. Both these sequences may prevent transcription across the D-loop region, since light and heavy strand transcription is terminated at CSB1 and coreTAS, respectively. Interestingly, the replication of the nascent D-loop strand, occurring in a direction opposite to that of heavy strand transcription, is also terminated at coreTAS, suggesting that coreTAS is involved in termination of both transcription and replication. Finally, we demonstrate that the loading of the helicase TWINKLE at coreTAS is reversible, implying that this site is a crucial component of a switch between D-loop formation and full-length mitochondrial DNA replication.

Computational design of orthogonal nucleoside kinases

We report the computational enzyme design of an orthogonal nucleoside analog kinase for 3'-deoxythymidine. The best kinase variant shows an 8500-fold change in substrate specificity, resulting from a 4.6-fold gain in catalytic efficiency for the nucleoside analog and a 2000-fold decline for the native substrate thymidine.

Directed evolution of an orthogonal nucleoside analog kinase via fluorescence-activated cell sorting

Nucleoside analogs (NAs) represent an important category of prodrugs for the treatment of viral infections and cancer, yet the biological potency of many analogs is compromised by their inefficient activation through cellular 2′-deoxyribonucleoside kinases (dNKs). We herein report the directed evolution and characterization of an orthogonal NA kinase for 3′-deoxythymidine (ddT), using a new FACS-based screening protocol in combination with a fluorescent analog of ddT. Four rounds of random mutagenesis and DNA shuffling of Drosophila melanogaster 2′-deoxynucleoside kinase, followed by FACS analysis, yielded an orthogonal ddT kinase with a 6-fold higher activity for the NA and a 20-fold k_cat/K_M preference for ddT over thymidine, an overall 10 000-fold change in substrate specificity. The contributions of individual amino acid substitutions in the ddT kinase were evaluated by reverse engineering, enabling a detailed structure–function analysis to rationalize the observed changes in performance. Based on our results, kinase engineering with fluorescent NAs and FACS should prove a highly versatile method for evolving selective kinase:NA pairs and for studying fundamental aspects of the structure–function relationship in dNKs.

Possible New Reaction Mechanisms of Dideoxynucleosides as Anti-Aids Drugs

Evidence is presented that a major class of drugs, the dideoxynucleosides (ddNs) and nucleoside/nucleotide analogues, may inhibit the symptoms of acquired immunodeficiency syndrome (AIDS) by initiation of inactivation at the ribonucleotide reductase (RNR) enzyme stage and/or inactivation of reverse transcriptase enzyme or at a stage more initial than that of the currently accepted DNA chain termination hypothesis. For example, it has been previously shown that ribonucleotide diphosphate reductase (RDPR) and ribonucleotide triphosphate reductase (RTPR) are inactivated with 2′-chloro-2 ‘-deoxyuridine 5′-diphosphate-([3′-3H]ClUDP) and triphosphate ([3′-3H]ClUTP) by reaction with an intermediate furanone, Scheme 2. RDPR has also been inactivated by 2‘-azido-2‘-deoxyuridine 5‘-diphosphate (N3UDP). Furthermore, addition of hydroxyurea to RNR can inhibit DNA synthesis which results in a rapid depletion of limiting deoxynucleotide triphosphate (dNTP) pools. There are similar perturbations of dNTP pools upon interaction of human RNR with 3‘-azido-2‘,3 ‘-dideoxythymidine (AZT), in human cell studies involving AZT/HIV and in adenosine/coformycin experiments in relation to inherited immunodeficiency, Table 1. Also, the herein proposed reduction mechanisms of nucleotides by RNR ( e.g., a single electron transfer from the nucleotide base to the phenol moiety of the tyrosyl radical of RNR via a pathway involving the thiyl radical of a cysteine residue) can also account for the chemistry of some antiretroviral drugs, the ddNs. Analyses are presented that the RNR reductions of regular unsubstituted nucleotides may occur predominantly via initial 2’ C-H abstraction instead of the originally proposed 3’ C-H abstraction mechanism. Also, it is noted that the fate of the phenol moiety of the tyrosyl unit in some RNR reactions with 2‘-halo-2‘-deoxynucleotides is not clear. The proposed reaction mechanisms may provide guidance for the development of potentially effective anti-AIDS drugs.

Establishment of a rodent model of HIV-associated sensory neuropathy

Human immunodeficiency virus (HIV)-associated sensory neuropathy (SN) is the most common neurological complication of HIV infection in the current highly active antiretroviral therapy era. The painful sensory neuropathy is associated with the use of dideoxynucleoside antiretrovirals, and its development limits the choice of antiretroviral drugs in affected patients. There are presently no effective therapies for HIV-SN, and moreover there has been no robust animal model of HIV-SN in which candidate therapeutic agents can be tested. In this paper, we show that we have established a rodent model of HIV-SN by oral administration of a dideoxynucleoside drug, didanosine, to transgenic mice expressing the HIV coat protein gp120 under a GFAP promoter. The neuropathy in these rodents is characterized by distal degeneration of unmyelinated sensory axons, similar to the "dying back" pattern of C-fiber loss seen in patients with HIV-SN. This model will be useful in examining mechanisms of distal axonal degeneration and testing potential neuroprotective compounds that may prevent development of the sensory neuropathy.

Altered pharmacokinetics of zalcitabine by concurrent use of NSAIDs in rats

AIM

To investigate the pharmacokinetic interactions between zalcitabine and nonsteroidal anti-inflammatory drugs (NSAIDs) in rats.

METHODS

Zalcitabine was administered to rats via an iv injection (20 mg/kg) in the presence or absence of ketoprofen or naproxen (20 mg/kg), and the pharmacokinetic parameters were determined by using non-compartmental analysis.

RESULTS

Compared with the control (zalcitabine alone), pretreatment with ketoprofen or naproxen 30 min prior to intravenous administration of zalcitabine significantly altered the pharmacokinetic profiles of zalcitabine in rats. Renal clearance of zalcitabine was reduced by approximately 3-4-fold in the presence of ketoprofen or naproxen. Consequently, systemic exposure (AUC) to zalcitabine in the rats pretreated with ketoprofen or naproxen was significantly greater than that for the control group given zalcitabine alone. The terminal plasma half-life of zalcitabine was also prolonged by 4-5-fold in the presence of ketoprofen or naproxen.

CONCLUSION

The NSAIDs ketoprofen and naproxen effectively altered the pharmacokinetics of zalcitabine. Therefore, concomitant use of ketoprofen or naproxen in patients being treated with zalcitabine may necessitate close monitoring for potential drug interactions.

Release of replication termination controls mitochondrial DNA copy number after depletion with 2',3'-dideoxycytidine

Although cellular mitochondrial DNA (mtDNA) copy number varies widely among cell lines and tissues, little is known about the mechanism of mtDNA copy number control. Most nascent replication strands from the leading, heavy-strand origin (O(H)) are prematurely terminated, defining the 3' boundary of the displacement loop (D-loop). We have depleted mouse LA9 cell mtDNA to approximately 20% of normal levels by treating with 2',3'-dideoxycytidine (ddC) and subsequently allowed recovery to normal levels of mtDNA. A quantitative ligation-mediated PCR assay was used to determine the levels of both terminated and extended nascent O(H) strands during mtDNA depletion and repopulation. Depleting mtDNA leads to a release of replication termination until mtDNA copy number approaches a normal level. Detectable total nascent strands per mtDNA genome remain below normal. Therefore, it is likely that the level of replication termination plays a significant role in copy number regulation in this system. However, termination of D-loop strand synthesis is persistent, indicating formation of the D-loop structure has a purpose that is required under conditions of rapid recovery of depleted mtDNA.

Phosphorylation of nucleoside analog antiretrovirals: a review for clinicians

Nucleoside analogs (zidovudine, didanosine, zalcitabine, stavudine, abacavir, lamivudine) have been administered as antiretroviral agents for more than a decade. They undergo anabolic phosphorylation by intracellular kinases to form triphosphates, which inhibit human immunodeficiency virus replication by competitively inhibiting viral reverse transcriptase. Numerous methods are used to elucidate the intracellular metabolic pathways of these agents. Intracellular and extracellular factors affect intracellular phosphorylation. Lack of standardization and complexity of methods used to study phosphorylation in patients limit interpretation of study results and comparability of findings across studies. However, in vitro and in vivo studies give important insights into mechanisms of action, metabolic feedback mechanisms, antiviral effects, and mechanisms of toxicity, and have influenced dosing regimens of nucleoside analogs.

3'-Azido-3'-deoxythymidine reduces the rate of transferrin receptor endocytosis in K562 cells

K562 cells, exposed for at least 24 h to 5 microM 3'-azido-3'-deoxythymidine (AZT), gave rise to an overall increase in the number of cell surface transferrin binding receptors (18-20%). This effect was ascertained either with binding experiments by using 125I-transferrin and with immunoprecipitation by using a specific monoclonal antibody against the transferrin receptor. At higher AZT concentrations (20 and 40 microM), a further increase was found, that is, up to 23% by binding experiments and up to 110% by immunoprecipitation. However, Scatchard analysis of the binding data indicated that although the number of cell surface transferrin receptors increased, the affinity of transferrin for its receptor did not change (Ka=4.0x108 M). Surprisingly, immunoprecipitation of total receptor molecules showed that the synthesis of receptor was not enhanced by the drug treatment. The effect of AZT on transferrin internalization and receptor recycling was also investigated. In this case, data indicated that the increase in the number of receptors at the cell surface was probably due to a slowing down of endocytosis rate rather than to an increased recycling rate of the receptor to cell surface. In fact, the time during which half the saturated amount of transferrin had been endocytosed (t1/2) was 2.15 min for control cells and 3.41, 3.04, and 3.74 min for 5, 20, and 40 microM AZT-treated cells, respectively. Conversely, recycling experiments did not show any significant differences between control and treated cells. A likely mechanism through which AZT could interfere with the transferrin receptor trafficking, together with the relevance of our findings, is extensively discussed.

Intracellular metabolism of 3'-azido-3'-deoxythymidine (AZT): a nuclear magnetic resonance study on T-lymphoblastoid cell lines with different resistance to AZT

This paper reports the results of 31P and 1H nuclear magnetic resonance (NMR) studies on the uptake and phosphorylation of 3'-azido-3'-deoxythymidine (AZT) in the human CD4+ T-lymphoblastoid cell line CCRF-CEM (CEM-1.3) and in its AZT-resistant cell variant MT-500, isolated by prolonged culturing of CEM cells in the presence of increasing AZT concentrations. After 3 hr of incubation in the presence of 0.5 mM AZT, both AZT and its monophosphorylated form (AZT-MP) could be detected in the sensitive cell line in concentrations above the NMR detection levels. In another cell line, MOLT-4, which is less sensitive to AZT effects, the intracellular level of AZT-MP was much lower and was only slightly raised by increasing the concentration of AZT in the extracellular and intracellular compartments. In the AZT-resistant clone MT-500, characterized by a very low thymidine kinase (TK, EC 2.7.1.21) activity with respect to the parental clone, the intracellular AZT-MP concentration was below detection (<0.02 nmol/10(6) cells). Since, however, not only AZT-MP but also AZT signals failed to be detected in MT-500 extracts following cell incubation with AZT, it was concluded that a TK deficiency cannot be the exclusive mechanism of AZT resistance in these cells. The possible effects of additional mechanisms of drug resistance, such as specific AZT cell extrusion and limited permeation, are discussed, together with the new prospects offered by NMR spectroscopy to further evaluate the limiting steps for the utilization of antiretroviral nucleoside analogues.

The existence of human immunodeficiency virus resistance to nucleoside-analog drugs has not been shown

Human immunodeficiency virus is thought to develop resistance to the common nucleoside-analog drugs by undergoing certain mutations in the pol gene. However, laboratory techniques which test for viral sensitivity to the drugs fail to provide direct evidence for the existence of the phenomenon. Available clinical and laboratory evidence indicates that drug resistance is entirely an artifact of the laboratory and most likely does not take place under natural circumstances. A method to directly confirm or disprove this notion is outlined.

Immunotoxicity of 2',3'-dideoxyinosine in female B6C3F1 mice

2',3'-dideoxyinosine (ddI) is one of several purine analogues used for the treatment of HIV and the acquired immunodeficiency syndrome (AIDS). These nucleoside analogues are promising in their inhibition of viral reverse transcriptase and termination of DNA synthesis. However, each of these drugs has toxicity associated with its use. A previous immunotoxicological evaluation of 2',3'-dideoxyadenosine (ddA), the parent compound of ddI, showed that ddA suppresses humoral immunity. These studies were undertaken to determine the potential for immunotoxicity due to treatment with ddI. This evaluation included an assessment of innate and acquired immunity after exposure to ddI (100, 250, 500, and 1000 mg/kg/day) for 14, 28 or 180 days. There were no overt signs of toxicity related to treatment with ddI except for a decrease in body weight in the group treated with the highest dose of ddI for 180 days. Overall, 6 months of treatment with ddI showed minimal effects on specific organs with the exception of the spleen and thymus. ddI selectively targets the immune system, with assays that challenge humoral immunity being more affected than those testing cell-mediated immunity. Innate immunity was unaffected by ddI treatment. Cell-mediated immunity, as measured by proliferative response to allogeneic cells (MLR) and the T cell mitogen (Concanavalin A), was moderately suppressed. There were no ddI associated effects on NK function or macrophage function as measured by the vascular clearance rate and phagocytic uptake of the tissue macrophages. The most sensitive indicator of ddI-induced immunotoxicity is suppression of the response to the T-dependent antigen, sheep red blood cells (sRBC). The No Observable Adverse Effect Level (NOAEL) for toxicity to the immune system following 14 days of exposure to ddI is 250 mg/kg. A suppression of the humoral immune response was seen at the lowest dose tested after treatment for 28 and 180 days. Thus, the NOAEL for both of these treatment periods is below 100 mg/kg/day.

2'3'-Dideoxyinosine inhibits the humoral immune response in female B6C3F1 mice by targeting the B lymphocyte

2',3'-Dideoxyinosine (ddI) is a purine nucleoside analog currently being used for the treatment of HIV-positive individuals and patients with AIDS. Preliminary immunotoxicity studies have shown that a consequence of ddI treatment in female B6C3F1 mice is the inhibition of the humoral immune response. This effect was dose dependent in a range of 100 to 1000 mg/kg with a no observed adverse effect level of less than 100 mg/kg for a 28-day treatment period. These studies were undertaken to investigate the immune cell target of ddI and to determine the mechanism of this toxicity. B6C3F1 mice were treated with 1000 mg/kg/day by oral gavage for 28 days. The B lymphocyte was identified as the cellular target of ddI through separation-reconstitution experiments of the adherent and nonadherent cell populations and of the T and B lymphocyte populations. These studies revealed a deficit in the ability of the nonadherent cells from ddI-treated mice to mount a normal antibody response to sRBC. A further separation of the nonadherent cells into T and B cells revealed a decreased ability of ddI-treated B cells to develop specific humoral immunity. Additional studies were undertaken to determine the mechanism by which ddI is affecting the B cell. Surface marker analysis of splenocytes revealed no difference in the cell populations between vehicle- and ddI-treated mice. B cell proliferation was also unaffected as shown by incubation with either a polyclonal stimulator, lipopolysaccharide, or anti-IgM plus IL-4. These results indicate that the primary cellular target of ddI is the B lymphocyte.

Murine thymic lymphoma is associated with a species-specific hematopoietic progenitor cell subpopulation

Many strains of laboratory mouse are uniquely susceptible to the development of T cell lymphoma/leukemia, either spontaneously or as a result of chemical or radiation exposure. In contrast, T cell leukemias or lymphomas which are relatively uncommon in human populations, are not easily induced by radiation, and are not generally associated with chemotherapy or chemical exposure. Evidence is presented to suggest that differences in the susceptibility to the development of these malignancies is related to subtle but important variations in the regulation of hematopoietic stem cell differentiation between these two species.

2'3'-Dideoxycytidine-induced thymic lymphoma correlates with species-specific suppression of a subpopulation of primitive hematopoietic progenitor cells in mouse but not rat or human bone marrow

The nucleoside analogue, 2',3'-dideoxycytidine (ddC), is a potent inhibitor of HIV replication, and AIDS patients receiving ddC experience clinical improvement without significant hematologic toxicity. Repeated ddC administration (1,000 mg/kg per day) for 13 wk produces an increased incidence of thymic lymphoma in B6C3F1 mice. Previous studies reveal a common link between chemically induced and genetically associated models of mouse thymic lymphoma that involves a defect in a subpopulation of primitive hematopoietic progenitor cells. This defect is characterized by suppression of a subpopulation of IL-3-responsive cells and ablation of stem cell factor synergy with GM-CSF. The present study was undertaken to ascertain whether ddC produces the same pattern of bone marrow toxicity in mice, and whether this effect is observed in rat and human bone marrow. ddC exposure in vivo and in vitro produced a select suppression of murine CFU identical to that previously described for other models of mouse thymic lymphoma. In contrast, this selective CFU suppression was not observed in rat and human bone marrow or in CD34+ cells. These studies suggest that the mouse may not be a good predictive model for ddC hematotoxicity in humans and that susceptibility to the development of thymic lymphoma may be unique to the mouse.

A novel assay system for anti-human immunodeficiency virus type 1 (HIV-1) activity using a subclone of a monocytic cell line, U937

A subclonal cl.1-14 cell was established from a monocytic cell line U937 by a limiting dilution method. The anti-HIV-1 activity of some antiviral compounds was evaluated in HIV-1-infected cl.1-14 cells. The results demonstrated that although AZT was a potent inhibitor of HIV-1 replication in cl.1-14 cells, its 50% effective concentration (EC50) values was 80 times higher than that in HIV-1 infected MT-4 cells; the EC50 of AZT was 0.16 microM and 0.002 microM in cl.1-14 and MT-4 cells, respectively. In contrast, the anti-HIV-1 activity of ddA, ddI and ddC in cl.1-14 cells was comparable to that in MT-4 cells. The antiviral activity of nevirapine, dextran sulfate, curdlan sulfate and T22 did not differ significantly between the cl.1-14 and MT-4 cells. The antiviral activity of several compounds in the HIV-1-infected cl.1-14 cells was similar to that in the HIV-1JR-FL-infected human peripheral macrophages. Our results suggest that cl.1-14 cell cultures are very useful for estimating antiviral activity and more advantageous than the use of peripheral blood macrophages.

Identification of a pharmacophore for nucleoside analog inhibitors directed at HIV-1 reverse transcriptase

An 'active analog' approach to receptor mapping was used to identify a pharmacophore for a set of thymidine nucleoside analog inhibitors of HIV-1 reverse transcriptase. The preliminary results indicate that the O2, O4', and O5' atoms are capable of adopting a unique pharmacophoric pattern which may be the key to their recognition by reverse transcriptase.

Metabolism and mechanism of antiretroviral action of purine and pyrimidine derivatives

Unlike herpes viruses, human immunodeficiency virus and other retroviruses do not encode specific enzymes required for the metabolism of the purine or pyrimidine nucleotides to their corresponding 5'-triphosphates. Therefore, 2',3'-dideoxynucleosides and acyclic nucleoside phosphonates must be phosphorylated and metabolized by host cell kinases and other enzymes of purine and/or pyrimidine metabolism. Different animal species (or even different cell types within one animal species) may differ in the efficiency of conversion of these drugs to their antivirally active metabolite(s). Three 2',3'-dideoxynucleosides are officially licensed for clinical use [i.e., zidovudine (3'-azido-2',3'-dideoxythymidine, AZT), didanosine (2',3'-dideoxyinosine, DDI) and zalcitabine (2',3'-dideoxycytidine, DDC)]. A number of other 2',3'-dideoxynucleoside analogues [among them stavudine (2',3'-didehydro-2',3'-dideoxythymidine, D4T), 2',3'-dideoxy-3'-thiacytidine (3TC), 2',3'-dideoxy-5-fluoro-3'-thiacytidine (FTC) and the acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)adenine (PMEA)] are currently under clinical investigation and are candidate compounds for eventual licensing as anti-AIDS drugs. The metabolic pathways, antimetabolic effects and mechanism of antiviral action of these nucleoside analogues will be discussed.

Evaluation of the genotoxic potential of selected anti-AIDS treatment drugs at clinical doses in vivo in mice

Six anti-AIDS drugs were assessed for in vivo genotoxicity and cytotoxicity at human clinical doses with the mouse bone marrow micronucleus assay. These included four dideoxynucleosides (azidothymidine, dideoxycytidine, dideoxyadenosine, and dideoxyinosine), an anthracycline antibiotic (doxorubicin), and a chelating agent (D-penicillamine). Cytological analysis of the mouse bone marrow cells revealed: (i) The dideoxynucleosides and D-penicillamine failed to induce significant number of micronuclei, and except for one of the five doses of dideoxyinosine, none of the dideoxynucleosides were cytotoxic at the doses tested. (ii) Doxorubicin induced micronuclei in a dose-dependent manner which was statistically significant at 4-times the clinical dose but was not cytotoxic at any of the doses tested.

Inhibition of HIV-1 replication in cultured cells with phosphorylated dideoxyuridine derivatives encapsulated in immunoliposomes

Among the 2',3'-dideoxynucleoside 5'-triphosphates containing a physiological base, 2',3'-dideoxyuridine 5'-triphosphate (ddUTP) has been reported to be among the most powerful inhibitors of human immunodeficiency virus (HIV) reverse transcriptase (RT) in cell-free systems. However, in contrast to other dideoxynucleosides, 2',3'-dideoxyuridine (ddU) is inactive in treatment of HIV-infected cells in culture, since it is a poor substrate for cellular nucleoside kinases. This problem cannot be overcome by the use of phosphorylated ddU because such compounds are unable to cross cell membranes. To promote entry and thus bypass the limiting steps of intracellular phosphorylation, we have encapsulated mono- and tri-phosphorylated ddU in liposomes coupled to monoclonal antibodies (immunoliposomes). We investigated antiviral effects in two human T cell lines (MT-4, CEM). We observed that ddU nucleotides remain phosphorylated for several weeks after encapsulation in immunoliposomes, and potent antiviral activity is obtained when these drugs are delivered into infected cells by cell-specific antibodies (ED50 < or = 1 microM on CEM). In contrast, no inhibition was observed with non-targeted liposomes containing phosphorylated ddU, or with empty liposomes, whether targeted or not.

Antiviral agents: characteristic activity spectrum depending on the molecular target with which they interact

The target protein (enzyme) with which antiviral agents interact determines their antiviral activity spectrum. Based on their activity spectrum, antiviral compounds could be divided into the following classes: (1) sulfated polysaccharides (i.e., dextran sulfate), which interact with the viral envelope glycoproteins and are inhibitory to a broad variety of enveloped viruses (i.e., retro-, herpes-, rhabdo-, and arenaviruses): (2) SAH hydrolase inhibitors (i.e., neplanocin A derivatives), which are particularly effective against poxvirus, (-)RNA viruses (paramyxovirus, rhabdovirus), and (+/-)RNA virus (reovirus); (3) OMP decarboxylase inhibitors (i.e., pyrazofurin) and CTP synthetase inhibitors (i.e., cyclopentenylcytosine), which are active against a broad range of DNA, (+)RNA, (-)RNA, and (+/-)RNA viruses; (4) IMP dehydrogenase inhibitors (i.e., ribavirin), which are also active against various (+)RNA and (-)RNA viruses and, in particular, ortho- and paramyxoviruses; (5) acyclic guanosine analogs (i.e., ganciclovir) and carbocyclic guanosine analogs (i.e., cyclobut-G), which are particularly active against herpesviruses (i.e., HSV-1, HSV-2, VZV, CMV); (6) thymidine analogs (i.e., BVDU, BVaraU), which are specifically active against HSV-1 and VZV because of their preferential phosphorylation by the virus-encoded thymidine kinase; (7) acyclic nucleoside phosphonates (i.e., HPMPA, HPMPC, PMEA, FPMPA), which, depending on the structure of the acyclic side chain, span an activity spectrum from DNA viruses (papova-, adeno-, herpes-, hepadna-, and poxvirus) to retroviruses (HIV); (8) dideoxynucleoside analogs (i.e., AZT, DDC), which act as chain terminators in the reverse transcriptase reaction and thus block the replication of retroviruses as well as hepadnaviruses; and (9) the TIBO, HEPT, and other TIBO-like compounds, which interact specifically with the reverse transcriptase of HIV-1 and thus block the replication of HIV-1, but not of HIV-2 or any other retrovirus.

Didanosine, a new antiretroviral drug. A review

In this article the literature about didanosine, an antiretroviral drug, is reviewed. The mechanism of action, biochemical pharmacology, pharmacokinetics, and clinical results of phase-I trials are discussed. Serious adverse effects such as pancreatitis and peripheral neuropathy have occurred in these trials. An antiretroviral effect was observed in terms of an increase in CD4+ lymphocytes and a decrease in p24 antigen levels in HIV-infected individuals. Didanosine seems to be a promising drug against HIV infection, but knowledge about its clinical efficacy is scanty.

Major growth reduction and minor decrease in mitochondrial enzyme activity in cultured human muscle cells after exposure to zidovudine

Zidovudine-induced mitochondrial myopathy in AIDS patients reported recently might be due to inhibition of mitochondrial DNA polymerase gamma. We investigated the effect of zidovudine on proliferation, differentiation, activity of mitochondrial- and nuclear-encoded enzymes, and mitochondrial DNA (mtDNA), in cultured human muscle cells. Marked inhibition of cell proliferation was found, even in the presence of low (10 mumol/L) zidovudine concentrations. Enzyme activity of the nuclear-encoded mitochondrial citrate synthase was not affected, and the partially mitochondrial-encoded cytochrome c oxidase was not decreased, except only after exposure to high concentrations (5 mmol/L) zidovudine. No decrease of mtDNA content and no mtDNA deletions were found in zidovudine-exposed muscle cells. We propose that the effect of zidovudine on muscle, seen in zidovudine-treated AIDS patients, results mainly from decrease in proliferation of muscle cells rather than inhibition of mtDNA replication.

Antiviral therapy: current concepts and practices

Drugs capable of inhibiting viruses in vitro were described in the 1950s, but real progress was not made until the 1970s, when agents capable of inhibiting virus-specific enzymes were first identified. The last decade has seen rapid progress in both our understanding of antiviral therapy and the number of antiviral agents on the market. Amantadine and ribavirin are available for treatment of viral respiratory infections. Vidarabine, acyclovir, ganciclovir, and foscarnet are used for systemic treatment of herpesvirus infections, while ophthalmic preparations of idoxuridine, trifluorothymidine, and vidarabine are available for herpes keratitis. For treatment of human immunodeficiency virus infections, zidovudine and didanosine are used. Immunomodulators, such as interferons and colony-stimulating factors, and immunoglobulins are being used increasingly for viral illnesses. While resistance to antiviral drugs has been seen, especially among AIDS patients, it has not become widespread and is being intensely studied. Increasingly, combinations of agents are being used: to achieve synergistic inhibition of viruses, to delay or prevent resistance, and to decrease dosages of toxic drugs. New approaches, such as liposomes carrying antiviral drugs and computer-aided drug design, are exciting and promising prospects for the future.

Molecular structures of two new anti-HIV nucleoside analogs: 9-(2,3-dideoxy-2-fluoro-beta-D-threo-pentofuranosyl)adenine and 9-(2,3-dideoxy-2-fluoro-beta-D-threo-pentofuranosyl)hypoxanthine

The x-ray crystal structures of two new anti-HIV compounds, 9-(2,3-dideoxy-2-fluoro-beta-D-threo-pentofuranosyl)adenine (2'-F-dd-araA) and 9-(2,3-dideoxy-2-fluoro-beta-D-threo- pentofuranosyl)hypoxanthine (2'-F-dd-aral), have been determined at two temperatures. Both crystals are in the space group P2(1)2(1)2(1), and their structures were solved by direct methods. Least-squares refinement produced final R-factors of 0.027 for the 2'-F-dd-araA structure and of 0.044 for the 2'-F-dd-aral structure, respectively. The latter structure contains a two-fold disordered conformation of the sugar moiety. All three conformers (one for 2'-F-dd-araA and two for 2'-F-dd-aral) adopt an anti chi CN glycosyl torsion angle. The sugar in the 2'-F-dd-araA structure has a C2'-endo pucker conformation, whereas the sugar in the 2'-F-dd-aral structure has a mixture of C2'-endo and C3'-endo pucker conformations. When the sugar adopts the C2'-endo conformation, the torsion angle about the C4'-C5' bond is in a transgauche+ conformation. In contrast, when the sugar adopts the C3'-endo conformation, the torsion angle about the C4'-C5' bond is in a gauche(+)-gauche- conformation. The C2'-F bond distance is 1.406(3) A, similar to that found in other aliphatic C-F bonds. The results suggest that the 2'-fluoro-2',3'-dideoxyarabinosyl nucleosides do not have a strong preference for either C2'-endo or C3'-endo sugar pucker.

Correlations between the in vitro and in vivo activity of anti-HIV agents: implications for future drug development

Some 10 years after the first recognition of acquired immunodeficiency syndrome (AIDS) as a new syndrome, we have identified a number of molecular targets to interrupt the replicative cycle of human immunodeficiency virus (HIV), the causative agent. A number of dideoxynucleosides have been identified as having anti-HIV activity in vitro, and several of these have been found to have clinical activity in patients. In contrast, while a number of agents have been found to block viral binding to the target cell in vitro, these agents have generally not shown clear-cut evidence of clinical activity. Agents which act at a variety of steps in the HIV replicative cycle are now under development, and it is likely that we will have an increased armamentarium to fight this disease in the near future.

Intracellular metabolism of 3'-azidothymidine in isolated human peripheral blood mononuclear cells

Azidothymidine (AZT) inhibits the replication of human immunodeficiency virus and it is the only drug so far licensed for treatment of acquired immunodeficiency syndrome (AIDS). A prerequisite for its antiviral activity is phosphorylation by cellular nucleoside kinases to the mono-, di-, and triphosphate levels. This study determined the capacity of isolated peripheral blood mononuclear cells (PBMC), resting or mitogen stimulated, from 36 different people of whom 5 were HIV+, to phosphorylate and accumulate intracellular AZT nucleotides. We found a large variation in the amount of products formed between PBMC treated at different times from the same individual as well as between the PBMC from different individuals. Resting PBMC showed the largest interindividual variation and their levels of AZT nucleotides were about 60-150-fold lower than in activated PBMC. The intracellular half lives of azidothymidine mono-, di-, and triphosphates, constituting, on the average, 96-99.2, 0.7-1.8, and 0.4-2.7% of total nucleotides at 0.08-1.6 microM AZT, respectively, were also determined. In mitogen-stimulated PBMC it was approximately 2.5 +/- 0.6 h for all the azidothymidine metabolites. The half-life for intracellular azidothymidine monophosphate in resting PBMC from two individuals was determined to 1.5 +/- 0.2 h. There appeared to be no significant difference in the AZT metabolism in PBMC from HIV-positive or-negative persons. A relative decrease in the intracellular formation of AZTDP and AZTTP from AZTMP was observed at concentrations of AZTMP above 1 microM. This fact may explain why lowering the doses of AZT still gives therapeutically efficient levels of the active metabolite AZTTP.

Comparative inhibition of hepatitis B virus DNA polymerase and cellular DNA polymerases by triphosphates of sugar-modified 5-methyldeoxycytidines and of other nucleoside analogs

Of a series of 14 nucleoside 5'-triphosphates, those of 2',3'-dideoxy-3'-fluoro-5-methylcytidine, 3'-azido-2',3'-dideoxy-5-methylcytidine, 2',3'-dideoxy-3'-fluoroguanosine, 2',3'-didehydro-2',3'-dideoxy-5-methylcytidine, 2',3'-dideoxy-3'-fluoro-5-ethylcytidine, and 2',3'-dideoxy-3'-fluoroadenosine emerged as the most potent inhibitors of hepatitis B virus DNA polymerase (50% inhibitory dose, 0.03 to 0.35 microM). In contrast, cellular DNA polymerases proved to be resistant to (alpha) or partially affected by (beta) these analogs. These compounds are among the most effective and selective inhibitors of endogenous hepatitis B virus DNA polymerase recognized to date.

Zidovudine myopathy: a distinctive disorder associated with mitochondrial dysfunction

Muscle biopsy specimens were obtained from 48 human immunodeficiency virus-infected patients suffering from various neuromuscular symptoms. Microscopic examination by conventional and electron microscopy revealed a characteristic structural myopathy associated with mitochondrial changes in 13 patients, all of whom had received long-term zidovudine therapy. The mean cumulative dose they had received (498 +/- 145 gm) was significantly higher than that of the other 14 zidovudine recipients of the study. They suffered from a progressive, usually painful, proximal myopathy with pronounced wasting, normal-to-moderately elevated creatine kinase levels, and a myopathic electromyographic pattern. The condition usually improved after withdrawal of the drug. Assay of mitochondrial enzymes, including succinate-cytochrome c reductase, cytochrome c oxidase, and citrate synthase, showed a decline in respiratory chain capacity. Southern blot analysis of mitochondrial DNA showed no abnormality. It is likely that mitochondrial dysfunction, probably resulting from drug-induced inhibition of the mitochondrial DNA polymerase, is implicated in the pathogenesis of this complication of zidovudine therapy.

Carbovir: the (-) enantiomer is a potent and selective antiviral agent against human immunodeficiency virus in vitro

In this paper we describe the in vitro antiviral activity of the (-) enantiomer of carbocyclic 2',3'-deoxydidehydroguanosine, (-) carbovir, a nucleoside analogue that has selective and potent anti-HIV activity in a series of lymphocyte culture systems. The cellular cytotoxicity of this compound has also been evaluated in a number of systems and compared to the saturated dideoxynucleoside analogues AZT and ddC.

Disposition and metabolism in mice of the potential antitumor and anti-human immunodeficiency virus-1 agent, 2-chloro-2',3'-dideoxyadenosine

A high-performance liquid chromatographic (HPLC) procedure was developed to examine the preclinical pharmacology and pharmacokinetics of 2-chloro-2',3'-dideoxyadenosine (ClddAd). The HPLC assay for ClddAd in human plasma was linear from 0.25 to 500 micrograms ClddAd/ml. Coefficients of variation for the measurement of ClddAd in human plasma were 9.7%, 4.1%, and 2.7% at 2.5, 25, and 250 micrograms/ml, respectively. Binding of ClddAd to human and mouse plasma proteins was determined by filtration to be 26.9% and 34.4%, respectively. ClddAd concentrations decreased by less than 5% when ClddAd was stored for 126 h at 37 degrees C in 0.9% NaCl or 0.1 M NaH2PO4 (pH 7.4) or when ClddAd was stored for 24 h at 37 degrees C in citrate-buffered human blood or plasma. Estimates of the lethal dose for 50% (LD50) and 10% (LD10) of male CD2F1 mice that received a single i.v. dose of ClddAd were 27 and 24 mg/kg, respectively. Elimination of a 24-mg/kg i.v. bolus dose of ClddAd from mouse plasma was biphasic, with half-lives of 0.73 and 14.7 min. The apparent volume of distribution of ClddAd was 215 ml/kg and the total body clearance was 20 ml min-1 kg-1. No ClddAd metabolites were detected in mouse plasma after in vivo exposure or in human whole blood or plasma after in vitro incubation. ClddAd was detected in the urine of mice within 2 min after exposure, and the total urinary excretion of unchanged ClddAd for 24 h after exposure to 24 mg/kg was 3.4% of the delivered dose. At least two possible ClddAd metabolites were detected in mouse urine; they did not co-elute with 2-chloro-2',3'-dideoxyinosine,2-chloradenine, or 2-chlorohypoxanthine.

Regulation of de novo and salvage pathways in chemotherapy

An overview was presented of our approach of inhibition of de novo and salvage pathways in pyrimidine and purine metabolism. 1. Combination of acivicin, an inhibitor of de novo biosynthesis, and dipyridamole, a transport inhibitor, provided synergistic cytotoxicity in hepatoma and colon carcinoma cells. 2. AZT, a competitive inhibitor of the salvage enzyme, thymidine kinase, and 5-FU or MTX provided synergistic cytotoxicity in hepatoma 3924A. In human colon carcinoma HT-29 cells AZT and methotrexate yielded synergistic cytotoxicity and thymidine and hypoxanthine together provided protection from the action of these drugs. 3. These observations are significant because in rat hepatoma 3924A and in human cell lines HT-29, HL-60 and K562 thymidine kinase activity was 16- to 67-fold higher than that of dTMP synthase. Therefore, inhibition of dTMP synthase activity alone may provide poor responses because the salvage pathways can circumvent this block. 4. In leukemic patients treated with tiazofurin, an inhibitor of IMP dehydrogenase, the rate-limiting enzyme of GTP biosynthesis, and with allopurinol, which inhibits GPRT activity through raising plasma hypoxanthine levels, synergistic therapeutic results were obtained. The responses in sensitive patients entailed a decrease in IMP dehydrogenase activity and GTP concentration in leukemic cells and down-regulation of the ras and myc oncogenes. The down-regulation of the ras oncogene by tiazofurin through the decrease of GTP concentration has now been shown in K562, HL-60 and hepatoma cells and in patients with chronic granulocytic leukemia in blast crisis. Tiazofurin may be useful in studies on selective depression of the expression of the ras oncogene. 5. In 27 consecutive patients 50% responded positively to tiazofurin treatment. From this group, 10 out of 12 patients (83%) with chronic granulocytic leukemia in blast crisis responded to tiazofurin treatment.

Initial clinical experience with dideoxynucleosides as single agents and in combination therapy

Several dideoxynucleosides, including 3'-azido-2',3'-dideoxythymidine (zidovudine, azidothymidine, AZT), 2',3'-dideoxycytidine (ddC), and 2',3'-dideoxyinosine (ddI), have been shown to be potent inhibitors of human immunodeficiency virus (HIV) replication in human T cells and macrophages. These compounds undergo anabolic phosphorylation within target cells to a 3'-triphosphate moiety; as triphosphates, they act at the level of HIV DNA polymerase (reverse transcriptase). AZT has been shown to reduce the morbidity and mortality of patients with severe HIV infection and to at least temporarily ameliorate certain cases of HIV-induced dementia. In phase 1 studies, ddC and ddI have been shown to induce immunologic and virologic improvements in patients with AIDS or related disorders; phase 2 studies of ddC and ddI are underway. The use of these drugs can be associated with toxicity. AZT can cause bone marrow toxicity or myositis with prolonged use, ddC can cause peripheral neuropathy at high doses, and ddI can cause sporadic pancreatitis and peripheral neuropathy at high doses. For each compound, however, a therapeutic window exists in which an anti-HIV effect can be attained without short-term toxicity in most patients. Dose-intensity appears to be an important determinant of the toxicity of dideoxynucleosides. Studies are underway to explore how the therapeutic profiles of these compounds may be enhanced by attention to scheduling or through the use of combination therapy.