Oral pharmacokinetics and efficacy of oral phospholipid remdesivir nucleoside prodrugs against SARS-CoV-2 in mice.

Oral broad-spectrum antivirals are urgently needed for the treatment of many emerging and contemporary RNA viruses. We previously synthesized 1-O-octadecyl-2-O-benzyl-sn-glyceryl-P-RVn (ODBG-P-RVn, V2043), a phospholipid prodrug of GS-441524 (remdesivir nucleoside, RVn), and demonstrated its in vivo efficacy in a SARS-CoV-2 mouse model. Structure-activity relationship studies focusing on the prodrug scaffold identified two modifications, 3-fluoro-4-methoxy-benzyl (V2053) and 4-cyano-benzyl (V2067), that significantly enhanced the in vitro broad-spectrum antiviral activity against multiple RNA viruses when compared to V2043. Here, we demonstrate that V2043, V2053, and V2067 are all orally bioavailable, well-tolerated, and achieve high sustained plasma levels after single oral daily dosing. All three phospholipid prodrugs are significantly more active than RVn in vitro and significantly reduce SARS-CoV-2 lung titers in prophylaxis and treatment mouse models of SARS-CoV-2 B.1.351 infection. On a molar basis, V2043 and V2067 are substantially more active than obeldesivir/GS-5245 and molnupiravir in vivo. Together, these data support the continued development of phospholipid RVn prodrugs for the treatment of SARS-CoV-2 and other RNA viruses of clinical concern.

Enhanced broad spectrum in vitro antiviral efficacy of 3-F-4-MeO-Bn, 3-CN, and 4-CN derivatives of lipid remdesivir nucleoside monophosphate prodrugs.

Broad spectrum oral antivirals are urgently needed for the early treatment of many RNA viruses of clinical concern. We previously described the synthesis of 1-O-octadecyl-2-O-benzyl-glycero-3-phospho-RVn (V2043), an orally bioavailable lipid prodrug of remdesivir nucleoside (RVn, GS-441524) with broad spectrum antiviral activity against viruses with pandemic potential. Here we compared the relative activity of V2043 with new RVn lipid prodrugs containing sn-1 alkyl ether or sn-2 glycerol modifications. We found that 3-F-4-MeO-Bn, 3-CN-Bn, and 4-CN-Bn sn-2 glycerol modifications improved antiviral activity compared to V2043 when tested in vitro against clinically important RNA viruses from 5 virus families. These results support the continued development of V2043 and sn-2 glycerol modified RVn lipid prodrugs for the treatment of a broad range of RNA viruses for which there are limited therapies.

1-O-Octadecyl-2-O-benzyl-sn-glyceryl-3-phospho-GS-441524 (V2043). Evaluation of Oral V2043 in a Mouse Model of SARS-CoV-2 Infection and Synthesis and Antiviral Evaluation of Additional Phospholipid Esters with Enhanced Anti-SARS-CoV-2 Activity.

Early antiviral treatments, including intravenous remdesivir (RDV), reduce hospitalization and severe disease caused by COVID-19. An orally bioavailable RDV analog may facilitate earlier treatment of non-hospitalized COVID-19 patients. Here we describe the synthesis and evaluation of alkyl glyceryl ether phosphodiesters of GS-441524 (RVn), lysophospholipid analogs which allow for oral bioavailability and stability in plasma. Oral treatment of SARS-CoV-2-infected BALB/c mice with 1-O-octadecyl-2-O-benzyl-sn-glyceryl-3-phospho-RVn (60 mg/kg orally, once daily for 5 days starting 12h after infection) reduced lung viral load by 1.5 log10 units versus vehicle at day 2 and to below the limit of detection at day 5. Structure/activity evaluation of additional analogs that have hydrophobic ethers at the sn-2 of glycerol revealed improved in vitro antiviral activity by introduction of a 3-fluoro-4-methoxy-substituted benzyl or a 3- or 4-cyano-substituted benzyl. Collectively, our data support the development of RVn phospholipid prodrugs as oral antiviral agents for prevention and treatment of SARS-CoV-2 infections.

Broad-Spectrum In Vitro Antiviral Activity of ODBG-P-RVn: An Orally-Available, Lipid-Modified Monophosphate Prodrug of Remdesivir Parent Nucleoside (GS-441524).

The necessity for intravenous administration of remdesivir confines its utility for treatment of coronavirus disease 2019 (COVID-19) to hospitalized patients. We evaluated the broad-spectrum antiviral activity of ODBG-P-RVn, an orally available, lipid-modified monophosphate prodrug of the remdesivir parent nucleoside (GS-441524), against viruses that cause diseases of human public health concern, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). ODBG-P-RVn showed 20-fold greater antiviral activity than GS-441524 and had activity nearly equivalent to that of remdesivir in primary-like human small airway epithelial cells. Our results warrant in vivo efficacy evaluation of ODBG-P-RVn. IMPORTANCE While remdesivir remains one of the few drugs approved by the FDA to treat coronavirus disease 2019 (COVID-19), its intravenous route of administration limits its use to hospital settings. Optimizing the stability and absorption of remdesivir may lead to a more accessible and clinically potent therapeutic. Here, we describe an orally available lipid-modified version of remdesivir with activity nearly equivalent to that of remdesivir against emerging viruses that cause significant disease, including Ebola and Nipah viruses. Our work highlights the importance of such modifications to optimize drug delivery to relevant and appropriate human tissues that are most affected by such diseases.

Broad-spectrum in vitro antiviral activity of ODBG-P-RVn: an orally-available, lipid-modified monophosphate prodrug of remdesivir parent nucleoside (GS-441524).

The intravenous administration of remdesivir for COVID-19 confines its utility to hospitalized patients. We evaluated the broad-spectrum antiviral activity of ODBG-P-RVn, an orally available, lipid-modified monophosphate prodrug of the remdesivir parent nucleoside (GS-441524) against viruses that cause diseases of human public health concern, including SARS-CoV-2. ODBG-P-RVn showed 20-fold greater antiviral activity than GS-441524 and had near-equivalent activity to remdesivir in primary-like human small airway epithelial cells. Our results warrant investigation of ODBG-P-RVn efficacy in vivo.

Rethinking Remdesivir: Synthesis, Antiviral Activity, and Pharmacokinetics of Oral Lipid Prodrugs.

Remdesivir (RDV; GS-5734) is currently the only FDA-approved antiviral drug for the treatment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The drug is approved for use in adults or children 12 years or older who are hospitalized for the treatment of COVID-19 on the basis of an acceleration of clinical recovery for inpatients with this disease. Unfortunately, the drug must be administered intravenously, restricting its use to those requiring hospitalization for relatively advanced disease. RDV is also unstable in plasma and has a complex activation pathway which may contribute to its highly variable antiviral efficacy in SARS-CoV-2-infected cells. Potent orally bioavailable antiviral drugs for early treatment of SARS-CoV-2 infection are urgently needed, and several, including molnupiravir and PF-07321332, are currently in clinical development. We focused on making simple, orally bioavailable lipid analogs of remdesivir nucleoside (RVn; GS-441524) that are processed to RVn monophosphate, the precursor of the active RVn triphosphate, by a single-step intracellular cleavage. In addition to high oral bioavailability, stability in plasma, and simpler metabolic activation, new oral lipid prodrugs of RVn had submicromolar anti-SARS-CoV-2 activity in a variety of cell types, including Vero E6, Calu-3, Caco-2, human pluripotent stem cell (PSC)-derived lung cells, and Huh7.5 cells. In Syrian hamsters, oral treatment with 1-O-octadecyl-2-O-benzyl-glycero-3-phosphate RVn (ODBG-P-RVn) was well tolerated and achieved therapeutic levels in plasma above the 90% effective concentration (EC90) for SARS-CoV-2. The results suggest further evaluation as an early oral treatment for SARS-CoV-2 infection to minimize severe disease and reduce hospitalizations.

Rethinking Remdesivir: Synthesis, Antiviral Activity and Pharmacokinetics of Oral Lipid Prodrugs.

Remdesivir (RDV, GS-5734) is currently the only FDA-approved antiviral drug for the treatment of SARS CoV-2 infection. The drug is approved for use in adults or children 12-years or older who are hospitalized for the treatment of COVID-19 on the basis of an acceleration of clinical recovery for inpatients with this disease. Unfortunately, the drug must be administered intravenously, restricting its use to those requiring hospitalization for relatively advanced disease. RDV is also unstable in plasma and has a complex activation pathway which may contribute to its highly variable antiviral efficacy in SARS-CoV-2 infected cells. Potent orally bioavailable antiviral drugs for early treatment of SARS-CoV-2 infection are urgently needed and several including molnupiravir and PF-07321332 are currently in clinical development. We focused on making simple, orally bioavailable lipid analogs of Remdesivir nucleoside (RVn, GS-441524) that are processed to RVn-monophosphate, the precursor of the active RVn-triphosphate, by a single-step intracellular cleavage. In addition to high oral bioavailability, stability in plasma and simpler metabolic activation, new oral lipid prodrugs of RVn had submicromolar anti-SARS-CoV-2 activity in a variety of cell types including Vero E6, Calu-3, Caco-2, human pluripotent stem cell (PSC)-derived lung cells and Huh7.5 cells. In Syrian hamsters oral treatment with ODBG-P-RVn was well tolerated and achieved therapeutic levels in plasma above the EC90 for SARS-CoV-2. The results suggest further evaluation as an early oral treatment for SARS-CoV-2 infection to minimize severe disease and reduce hospitalizations.

Octadecyloxyethyl benzyl tenofovir: A novel tenofovir diester provides sustained intracellular levels of tenofovir diphosphate.

Pre-exposure prophylaxis (PrEP) with topically or systemically administered antiretroviral agents can prevent acquisition of human immunodeficiency virus type 1 (HIV-1) infection. However, in clinical trials using tenofovir-containing agents, HIV-1 acquisition is reduced but not eliminated. Incomplete adherence remains the major contributor to failure. Sustained release or long-acting antiretroviral agents may provide better HIV-1 protection by reducing the clinical impact of incomplete adherence. To reduce dosing frequency, we synthesized a novel tenofovir prodrug, octadecyloxyethyl benzyl tenofovir (ODE-Bn-TFV), that is designed to release TFV slowly in tissues, and showed potent anti-HIV activity in vitro (EC50 = 1.7 nM). In cells exposed to 14C labeled TFV, ODE-Bn-TFV or the quickly activated monoester ODE-TFV, rapid cellular uptake for both lipophilic analogs was noted, achieving 50-fold higher levels than unmodified TFV after 48 h. Following exposure to ODE-[8-14C]TFV, the intracellular diphosphate levels were approximately four-fold higher than with ODE-Bn-TFV. However, intracellular TFVpp drug levels fell rapidly yielding a half-life of about two days. TFVpp levels in ODE-Bn-TFV treated cells decreased much more slowly and reached half-maximal levels in about seven days. These results suggest early accumulation of ODE-Bn-TFV followed by sustained intracellular release following cleavage of the ester bonds linking the ODE and benzyl moieties to the active molecular precursor, thereby potentially allowing for less frequent administration than with more rapidly activated forms of tenofovir.

Octadecyloxyethyl Adefovir Exhibits Potent in vitro and in vivo Cytotoxic Activity and Has Synergistic Effects with Ara-C in Acute Myeloid Leukemia.

Acute myeloid leukemia (AML) continues to be a deadly disease, with only 50-70% of patients achieving complete remission and less than 30% of adults having sustained long-term remissions. In order to address these unmet medical needs, we carried out a high-throughput screen of an in-house library of on- and off-patent drugs with the OCI/AML-2 cell line. Through this screen, we discovered adefovir dipi-voxil (adefovir-DP) as being active against human AML. In addition to adefovir-DP, there are second-generation formulations of adefovir, including octadecyloxyethyl adefovir (ODE-adefovir) and hexadecyloxypropyl adefovir (HDP-adefovir), which were designed to overcome the pharmacokinetic problems of the parent compound adefovir. Given the known clinical benefit of nucleoside analogs for the treatment of AML, we undertook studies to evaluate the potential benefit of adefovir-based molecules. In AML cell lines and patient samples, adefovir-DP and ODE-adefovir were highly potent, whereas HDP-adefovir was significantly less active. Interestingly, ODE-adefovir was remarkably less toxic than adefovir-DP towards normal hematopoietic cells. In addition, ODE-adefovir at a dose of 15 mg/kg/day showed potent activity against human AML in a NOD/SCID mouse model, with a reduction of human leukemia in mouse bone marrow of > 40% in all mice tested within 20 days of treatment. Based on its chemical structure, we hypothesized that the cytotoxicity of ODE-adefovir toward AML was through cell cycle arrest and DNA damage. Indeed, ODE-adefovir treatment induced cell cycle arrest in the S phase and increased levels of pH2Ax, indicating the induction of DNA damage. Furthermore, there was an increase in phospho-p53, transactivation of proapoptotic genes and activation of the intrinsic apoptotic pathway. Subsequent investigation unveiled strong synergism between ODE-adefovir and ara-C, making their coadministration of potential clinical benefit. Expression of MRP4, a nucleoside transporter, appeared to influence the response of AML cells to ODE-adefovir, as its inhibition potentiated ODE-adefovir killing. Taken together, our findings indicate that clinical development of ODE-adefovir or related compounds for the treatment of AML is warranted.

Inhibition of adenovirus serotype 14 infection by octadecyloxyethyl esters of (S)-[(3-hydroxy-2-phosphonomethoxy)propyl]- nucleosides in vitro.

On September 22, 2008, a physician on Prince of Wales Island, Alaska, notified the Alaska Department of Health and Social Services (ADHSS) of an unusually high number of adult patients with recently diagnosed pneumonia (n = 10), including three persons who required hospitalization and one who died. ADHSS and CDC conducted an investigation to determine the cause and distribution of the outbreak, identify risk factors for hospitalization, and implement control measures. This report summarizes the results of that investigation, which found that the outbreak was caused by adenovirus 14 (Ad14), an emerging adenovirus serotype in the United States that is associated with a higher rate of severe illness compared with other adenoviruses. Among the 46 cases identified in the outbreak from September 1 through October 27, 2008, the most frequently observed characteristics included the following: male (70%), Alaska Native (61%), underlying pulmonary disease (44%), aged > or = 65 years (26%), and current smoker (48%). Patients aged > or = 65 years had a fivefold increased risk for hospitalization. The most commonly reported symptoms were cough (100%), shortness of breath (87%), and fever (74%). Of the 11 hospitalized patients, three required intensive care, and one required mechanical ventilation. One death was reported. Ad14 isolates obtained during the outbreak were identical genetically to those in recent community-acquired outbreaks in the United States which suggests the emergence of a new, and possibly more virulent Ad14 variant. Clinicians should consider Ad14 infection in the differential diagnosis for patients with community-acquired pneumonia, particularly when unexplained clusters of severe respiratory infections are detected.

A novel lipid prodrug strategy for sustained delivery of hexadecyloxypropyl 9-[2-(phosphonomethoxy)ethyl]guanine (HDP-PMEG) on unwanted ocular proliferation.

Proliferative vitreoretinopathy (PVR) is a blinding eye disease and there is no effective pharmacological measure to prevent PVR development. The difficulty comes from lack of potent antiproliferative agent and lack of sustained delivery to cover high-risk time window for PVR to develop. Lipid prodrug of PMEG, hexadecyloxypropyl 9-[(2-phosphonomethoxy)ethyl]guanine (HDP-PMEG), was prepared and was evaluated as a pharmacological adjuvant to surgical management of PVR. A dose-escalation study determined that the highest nontoxic dose for intravitreal use in pigmented rabbits was 3 µg per eye. The genotoxicity of HDP-PMEG was harnessed as a perioperative preventative measure against PVR in a rabbit eye model while the sustained intravitreal pharmacological effect was evaluated on a laser-induced fibrovascular model in rat eye. After intravitreal 3 µg, HDP-PMEG particles in the rabbit vitreous was visible for at least 6 weeks. A single 50-min intravitreal infusion of HDP-PMEG demonstrated significant inhibition of PVR formation when compared with the eyes infused with only BSS (BSS vs. HDP-PMEG: estimate = 1.14, OR = 3.1, p = .027). A single intravitreal 104 ng (equivalent to 3 µg for rabbit eye) of HDP-PMEG significantly inhibit laser-induced fibrovascular proliferation in rat eye by 55% (least square mean pixel, BSS = 4763569.5 vs. HDP-PMEG = 2148129.7, p < .0001, generalized estimating equation [GEE]). Retinal fluorescein angiography showed the odds for BSS intervened eyes to have higher-rated FA leaking grades were 38.5 times compared with HDP-PMEG treated eyes (p < .0001, GEE). Our study results indicate that single intravitreal HDP-PMEG may be a promising ocular drug delivery as a perioperative intervention to prevent PVR reoccurrence following primary surgical management.

Synthesis and Antiviral Evaluation of Octadecyloxyethyl Benzyl 9-[(2-Phosphonomethoxy)ethyl]guanine (ODE-Bn-PMEG), a Potent Inhibitor of Transient HPV DNA Amplification.

Human papillomavirus (HPV) high-risk genotypes such as HPV-16 and HPV-18 cause the majority of anogenital tract carcinomas, including cervical cancer, the second most common malignancy in women worldwide. Currently there are no approved antiviral agents that reduce or eliminate HPV and reverse virus-associated pathology. We synthesized and evaluated several alkoxyalkyl acyclic nucleoside phosphonate diesters and identified octadecyloxyethyl benzyl 9-[(2-phosphonomethoxy)ethyl]guanine (ODE-Bn-PMEG) as an active compound which strongly inhibited transient amplification of HPV-11, -16, and -18 origin-containing plasmid DNA in transfected cells at concentrations well below its cytotoxic concentrations. ODE-Bn-PMEG demonstrated increased uptake in human foreskin fibroblast cells and was readily converted in vitro to the active antiviral metabolite, PMEG diphosphate. The P-chiral enantiomers of ODE-Bn-PMEG were obtained and appeared to have equivalent antiviral activities against HPV. ODE-Bn-PMEG is a promising candidate for the local treatment of HPV-16 and HPV-18 and other high-risk types, an important unmet medical need.

Micelle formulation of hexadecyloxypropyl-cidofovir (HDP-CDV) as an intravitreal long-lasting delivery system.

There still is an unmet need for a safe and sustained intravitreal drug delivery system. In this study we are proposing and characterizing a micelle based, clear-media intravitreal drug delivery system using the lipid derivatized nucleoside analog, hexadecyloxypropyl-cidofovir (HDP-CDV, CMX 001). HDP-CDV forms micelles in water and in vitreous supernatant with the critical micelle concentration of 19 µg/mL and 9 µg/mL, respectively at 37 °C. The formed micelles had the average size of 274.7 nm and the Zeta potential of -47.1 mV. Drug release study in the excised rabbit vitreous showed a sustained release profile with a half-life of 2.7 days. The micelle formulation of HDP-CDV demonstrated a good safety profile in two animal species (rabbit and guinea pig) following intravitreal injection. The sustained efficacy was tested in a pretreatment study design and the drug potency was tested in an ongoing herpes simplex virus (HSV-1) retinitis model. The pretreatment studies using single intravitreal injection and later HSV-1 infection revealed at least 9 weeks of vitreous presence and therapeutic level of HDP-CDV, with 71% eyes protection from infection. The treatment study demonstrated that intravitreal administration halted active HSV-1 retinitis in 80% of the infected eyes while cidofovir (CDV) treatment failed to suppress active HSV-1 retinitis. In summary, lipid derivatized nucleoside analogs can be formulated as a micelle intravitreal injection and provides a sustained drug release in vitreous for chronic retinal diseases.

Novel mutations in the gene encoding very long-chain acyl-CoA dehydrogenase identified in patients with partial carnitine palmitoyltransferase II deficiency.

INTRODUCTION: Twenty-six patients with clinical symptoms of adult onset carnitine palmitoyltransferase II (CPTII) deficiency were examined. All patients had skeletal muscle CPTII enzyme activity levels indicative of heterozygosity for CPT2 mutations, however sequence analysis identified no pathogenic mutations within the CPT2 gene. METHODS: Because the reaction product of CPTII is the substrate for very long-chain acyl-CoA dehydrogenase (VLCAD), we examined the ACADVL gene in these patients by sequence analysis. RESULTS: Missense mutations within the ACADVL gene were identified in 3 of the patients. CONCLUSIONS: The locations of the altered amino acid residues within the crystal structure of VLCAD are on the surface of the molecule and may be involved in interactions with neighboring molecules. These findings support the importance of considering that mutations may be present in the ACADVL gene when a significant partial deficiency is found in CPTII activity, but no mutations in the CPT2 gene can be identified.

A novel cytarabine crystalline lipid prodrug: hexadecyloxypropyl cytarabine 3',5'-cyclic monophosphate for proliferative vitreoretinopathy.

PURPOSE: The objectives of this study were to synthesize and characterize two types of cytarabine (Ara-C) lipid produgs and evaluate the prodrugs for sustained intraocular delivery after administration by intravitreal injection. METHODS: Hexadecyloxypropyl cytarabine 5'-monophosphate (HDP-P-Ara-C) and hexadecyloxypropyl cytarabine 3',5'-cyclic monophosphate (HDP-cP-Ara-C) were synthesized starting from cytarabine (1-ß-D-arabinofuranosylcytosine). Their vitreal clearance profile was simulated using a custom dissolution chamber, in vitro cytotoxicity was evaluated using cell proliferation assays, and in vivo ocular properties in rat and rabbit eyes were assessed using biomicroscopy, indirect ophthalmoscopy, tonometry, electroretinography, and histology. RESULTS: HDP-P-Ara-C was cleared from the dissolution chamber (flow rate 2 µL/min) within 7 days. In contrast, HDP-cP-Ara-C, a much more insoluble prodrug, was still detectable 36 days after the dissolution process was started. HDP-P-Ara-C had a 50% cytotoxicity concentration of 52±2.6 µM in human retinal pigment epithelium (ARPE-19) and 32±2.2 µM in a rat Müller cell line, rMC-1. The 50% cytotoxicity concentration values for HDP-cP-Ara-C in ARPE-19 and rMC-1 cells were 50 µM and 25 µM, respectively. HDP-P-Ara-C was not detectable 2 weeks after the highest intravitreal dose (228 µg/rat eye) was injected, and no ocular toxicity was found. With HDP-cP-Ara-C, the drug depot was visible for 26 weeks following a single intravitreal injection (800 µg/rabbit eye). For both compounds, the electroretinogram, intraocular pressure, and other toxicity studies were negative except for the highest dose of HDP-cP-Ara-C (800 µg/eye), which had focal toxicity from the direct touch of the retina and decreased dark adapted a-waves and decreased flicker electroretinogram amplitudes (generalized estimating equations, p=0.039 and 0.01). CONCLUSIONS: The cyclic monophosphate prodrug, HDP-cP-Ara-C, was found to have physiochemical properties better suited for sustained delivery of cytarabine to posterior segments of the eye. These properties included limited aqueous solubility, in vitro antiproliferative activity, and good tolerability after injection into rabbit eyes.

Intraocular safety and pharmacokinetics of hexadecyloxypropyl-cidofovir (HDP-CDV) as a long-lasting intravitreal antiviral drug.

PURPOSE: To evaluate the intraocular safety and pharmacokinetics of hexadecyloxypropyl-cidofovir (HDP-CDV), the hydrolysis product of HDP-cyclic-CDV, a long-lasting intravitreal cidofovir prodrug for cytomegalovirus (CMV) retinitis. METHODS: HDP-cyclic-CDV was suspended in phosphate-buffered saline (PBS) at 37°C and formation of HDP-CDV was monitored by high-performance liquid chromatography (HPLC) analysis for 30 weeks. The safety and pharmacokinetics of HDP-CDV intravitreal injections were studied using New Zealand Red rabbits and (14)C labeled HDP-CDV. Ocular tissues from five time points (1, 3, 7, 14, and 35 days) were analyzed by scintillation counting and HPLC to characterize the pharmacokinetics. RESULTS: During the hydrolysis study, approximately 35% of the HDP-cyclic-CDV was converted to HDP-CDV. Evaluation of safety found no toxicity after intravitreal injection of HDP-CDV up to 28 µg/eye. Intravitreal pharmacokinetics of HDP-CDV in the retina, choroid, and vitreous followed a two-phase elimination process and elimination half-lives of 8.4 days (retina), 6.9 days (choroid), and 6.2 days (vitreous). In the retina, cidofovir and an unknown metabolite were detected in the first 2 weeks, and the maximum metabolite concentrations were present 48 hours after the maximum HDP-CDV concentration. CONCLUSIONS: HDP-cyclic CDV, under simulated physiologic conditions, slowly converts to HDP-CDV, another potent anti-CMV prodrug that may be taken up by retinal cells and metabolized further to the active antiviral metabolite, cidofovir diphosphate. Taken together, these observations help to explain the ability of a single intravitreal dose of HDP-cyclic-CDV to prevent viral retinitis for up to 68 days in a rabbit model.

Inhibition of HIV-1 by octadecyloxyethyl esters of (S)-[3-hydroxy-2-(phosphonomethoxy)propyl] nucleosides and evaluation of their mechanism of action.

(S)-1-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine (HPMPC [cidofovir]) and (S)-9-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine (HPMPA) are potent inhibitors of a variety of DNA viruses. These drugs possess a 3'-hydroxyl equivalent which could support chain extension from an incorporated drug molecule. HPMPC and HPMPA were initially reported to lack activity against human immunodeficiency virus type 1 (HIV-1); more recent results have shown that the octadecyloxyethyl (ODE) and hexadecyloxypropyl (HDP) esters of HPMPA are potent inhibitors of the virus. We have synthesized the ODE esters of a series of (S)-[3-hydroxy-2-(phosphonomethoxy)propyl] (HPMP) nucleosides, including HPMPC, HPMP-guanine (HPMPG), HPMP-thymine (HPMPT), and HPMP-diaminopurine (HPMPDAP), as well as the ODE ester of the obligate chain terminator (S)-9-[3-methoxy-2-(phosphonomethoxy)-propyl]adenine (MPMPA). All compounds except ODE-HPMPT were inhibitors of HIV-1 replication at low nanomolar concentrations. These compounds were also inhibitors of the replication of HIV-1 variants that are resistant to various nucleoside reverse transcriptase (RT) inhibitors at concentrations several times lower than would be expected to be achieved in vivo. To investigate the mechanism of the antiviral activity, the active metabolites of HPMPC and HPMPA were studied for their effects on reactions catalyzed by HIV-1 RT. Incorporation of HPMPC and HPMPA into a DNA primer strand resulted in multiple inhibitory effects exerted on the enzyme and showed that neither compound acts as an absolute chain terminator. Further, inhibition of HIV-1 RT also occurred when these drugs were located in the template strand. These results indicate that HPMPC and HPMPA inhibit HIV-1 by a complex mechanism and suggest that this class of drugs has a broader spectrum of activity than previously shown.

Application of kinase bypass strategies to nucleoside antivirals.

Nucleoside and nucleotide analogs have served as the cornerstones of antiviral therapy for many viruses. However, the requirement for intracellular activation and side-effects caused by distribution to off-target sites of toxicity still limit the efficacy of the current generation of drugs. Kinase bypass strategies, where phosphorylated nucleosides are delivered directly into cells, thereby, removing the requirement for enzyme catalyzed phosphorylation steps, have already changed the face of antiviral therapy in the form of the acyclic nucleoside phosphonates, cidofovir, adefovir (given orally as its dipivoxil prodrug) and tenofovir (given orally as its disoproxil prodrug), currently used clinically. These strategies hold further promise to advance the field of antiviral therapy with at least 10 kinase bypass and tissue targeted prodrugs, representing seven distinct prodrug classes, currently in clinical trials. This article reviews the history of kinase bypass strategies applied to nucleoside antivirals and the evolution of different tissue targeted prodrug strategies, highlighting clinically relevant examples.

Synthesis and antiviral evaluation of 9-(S)-[3-alkoxy-2-(phosphonomethoxy)propyl]nucleoside alkoxyalkyl esters: inhibitors of hepatitis C virus and HIV-1 replication.

We reported previously that octadecyloxyethyl 9-(S)-[3-hydroxy-2-(phosphonomethoxy)-propyl]adenine (ODE-(S)-HPMPA) was active against genotype 1b and 2a hepatitis C virus (HCV) replicons. This is surprising because acyclic nucleoside phosphonates have been regarded as having antiviral activity only against double stranded DNA viruses, HIV and HBV. We synthesized octadecyloxyethyl 9-(S)-[3-methoxy-2-(phosphonomethoxy)propyl]-adenine and found it to be active in genotype 1b and 2a HCV replicons with EC50 values of 1-2 µM and a CC50 of > 150 µM. Analogs with substitutions at the 3'-hydroxyl larger than methyl or ethyl, or with other purine bases were less active but most compounds had significant antiviral activity against HIV-1 in vitro. The most active anti-HIV compound was octadecyloxyethyl 9-(R)-[3-methoxy-2-(phosphonomethoxy)propyl]guanine with an EC50 < 0.01 nanomolar and a selectivity index of > 4.4 million.