Remofovir Mesylate: A Prodrug of PMEA with Improved Liver-Targeting and Safety in Rats and Monkeys

Stimulus-cleavable chemistry in the field of controlled drug delivery

This review comprehensively summarises stimulus-cleavable linkers from various research areas and their cleavage mechanisms, thus provides an insightful guideline to extend their potential applications to controlled drug release from nanomaterials.

Safety, pharmacokinetics and pharmacogenetics of a single ascending dose of pradefovir, a novel liver-targeting, anti-hepatitis B virus drug, in healthy Chinese subjects

BACKGROUND

Pradefovir is efficiently converted to adefovir [9-(2-phosphonylmethoxyethyl) adenine (PMEA)], producing high hepatic PMEA concentration but low levels in the systemic circulation and kidney. The aim of this study is to evaluate the tolerability, adverse effect (AEs), pharmacokinetics and pharmacogenetics of a single ascending dose of pradefovir.

METHODS

Fifty healthy subjects were divided into five groups and randomized within each group at a ratio of 3:1:1 to receive a single ascending dose of pradefovir (10, 30, 60, 90, or 120 mg), and 10 mg adefovir dipivoxil (ADP) or placebo. Blood and urine samples were collected and analyzed. A total of 1930 polymorphic loci were analyzed in 6 blood samples collected from the 90 mg pradefovir group.

RESULTS

The single oral dose of pradefovir up to 120 mg was well tolerated. A total of 29 dose-limited mild AEs were reported in 17 subjects. The peak plasma concentration (C _max) and area under the curve (AUC)_0-48 of serum pradefovir ranged from (21.41 ± 12.98) to (447.33 ± 79.34) ng/mL and (46.10 ± 29.45) to (748.18 ± 134.15) ng h/mL across the dose range, respectively. The C _max and AUC_0-48 of serum PMEA ranged from 18.10 ± 4.96 to 312.33 ± 114.19 ng/mL and 72.65 ± 28.25 to 1095.48 ± 248.47 ng h/mL. Generally, no kidney impairment was observed. Pharmacogenetic analysis identified three metabolism-related single nucleotide polymorphism (SNP) locis, P450 (cytochrome) oxidoreductase [POR (rs6965343)], arylamine N-acetyltransferases [NAT1 (rs4986993)] and CYP2F1 (rs305968)], and one distribution-related loci, orosomucoid 2 [ORM2 (rs12685968)].

CONCLUSIONS

The single oral dose of pradefovir 10-120 mg was well tolerated. SNPs may be associated with variable rates of adverse events.

TRIAL REGISTRATION NUMBER

CTR20140341.

Development and Validation of a Sensitive LC-MS-MS Method for the Determination of Adefovir in Human Serum and Urine: Application to a Clinical Pharmacokinetic Study

A rapid and sensitive liquid chromatography-tandem mass spectrometry method was developed and validated for the quantification of adefovir (PMEA,9-(2-phosphonylmethoxyethyl) adenine) concentration in human serum and urine. The analysis was performed on a negative ionization electrospray mass spectrometer via multiple reaction monitoring. The monitored transitions were set at m/z 272.0 → 134.0 and m/z 276.0 → 149.8 for PMEA and internal standard, respectively. After protein precipitation, samples were separated by high-performance liquid chromatography on a reversed-phase Dikma Diamonsil C18 (250 × 4.6 mm; 5 µm) column with a mobile phase of 0.1 mM ammonium formate buffer-methanol. The calibration curves were linear over the serum concentration range 0.5-1,000 ng/mL and urine concentration range 2.0-1,000 ng/mL. The intra- and interday precision values of PMEA in both serum and urine were lower than 18.16% for low quality control and 13.70% for medium and high quality control. The accuracy, recovery, matrix factor and stability were also within the acceptable limits. The developed method was successfully applied to the pharmacokinetic study of following oral administration of single dose of pradefovir mesylate (10, 30, 60, 90 and 120 mg) and adefovir dipivoxil (10 mg) to healthy Chinese volunteers.

The treatment of chronic hepatitis B: Focus on adefovir-like antivirals

Several options for the treatment of hepatitis B have been licensed in the last years: interferon, pegylated interferon, lamivudine, adefovir, entecavir, and telbivudine. In addition tenofovir has been licensed in the EU and is expected to be licensed in the USA in 2008. The antivirals can be divided into “lamivudine-like” and “adefovir-like”, which clinically differ in their capacity to induce “YMDD” mutants, which are the hallmark of lamivudine resistance. The differing resistance profile makes them good combination partners, even in the absence of synergy in antiviral potency.

Synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine

The synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine is reported. Initial incorporation of N,N-dibenzylformamidino protection of the C2-amino of guanosine masked the reactivity of that group and simplified purification of subsequent analogues. The first key intermediate, 9-(2,5-bis-O-tert-butyldimethylsilyl-beta-D-ribofuranosyl)-2-N-(N,N-dibenzylformamidino)guanine (3a), was prepared in 60% yield after recycling of the undesired 3',5'-bis-O-protected byproduct (4a) by simple equilibration in methanol to a mixture of the two bis-O-protected compounds. Thus, protected, the 3'-position was manipulated to form the 3'-deoxyribo- or 3'-deoxyxylo-3'-azido derivatives (9 or 16, respectively). Further selective manipulations provided the cis-5'-monophosphate (3-chlorophenyl)-1,3-propanyl diester prodrugs (HepDirect prodrugs), 15 and 21. These HepDirect prodrugs were demonstrated to activate to their respective NTPs in rat hepatocytes.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

Pradefovir: a prodrug that targets adefovir to the liver for the treatment of hepatitis B

Adefovir dipivoxil, a marketed drug for the treatment of hepatitis B, is dosed at submaximally efficacious doses because of renal toxicity. In an effort to improve the therapeutic index of adefovir, 1-aryl-1,3-propanyl prodrugs were synthesized with the rationale that this selectively liver-activated prodrug class would enhance liver levels of the active metabolite adefovir diphosphate (ADV-DP) and/or decrease kidney exposure. The lead prodrug (14, MB06866, pradefovir), identified from a variety of in vitro and in vivo assays, exhibited good oral bioavailability (F = 42%, mesylate salt, rat) and rate of prodrug conversion to ADV-DP. Tissue distribution studies in the rat using radiolabeled materials showed that cyclic 1-aryl-1,3-propanyl prodrugs enhance the delivery of adefovir and its metabolites to the liver, with pradefovir exhibiting a 12-fold improvement in the liver/kidney ratio over adefovir dipivoxil.

Metabolic activation of pradefovir by CYP3A4 and its potential as an inhibitor or inducer

Metabolic activation of pradefovir to 9-(2-phosphonylmethoxyethyl)adenine (PMEA) was evaluated by using cDNA-expressed CYP isozymes in portal vein-cannulated rats following oral administration and in human liver microsomes. The enzyme induction potential of pradefovir was evaluated in rats following multiple oral dosing and in primary cultures of human hepatocytes. The results indicated that CYP3A4 is the only cDNA-expressed CYP isozyme catalyzing the conversion of pradefovir to PMEA. Pradefovir was converted to PMEA in human liver microsomes with a K(m) of 60 microM, a maximum rate of metabolism of 228 pmol/min/mg protein, and an intrinsic clearance of about 359 ml/min. Addition of ketoconazole and monoclonal antibody 3A4 significantly inhibits the conversion of pradefovir to PMEA in human liver microsomes, suggesting the predominant role of CYP3A4 in the metabolic activation of pradefovir. Pradefovir at 0.2, 2, and 20 microM was neither a direct inhibitor nor a mechanism-based inhibitor of CYP3A4, CYP2D6, CYP2C9, CYP2C19, CYP2E1, and CYP1A2 in human liver microsomes. In rats, the liver was the site of metabolic activation of pradefovir, whereas the small intestine did not play a significant role in the metabolic conversion of pradefovir to PMEA. Daily oral dosing (300 mg/kg of body weight) to rats for 8 days showed that pradefovir was not an inducer of P450 enzymes in rats. Furthermore, pradefovir at 10 microg/ml was not an inducer of either CYP1A2 or CYP3A4/5 in primary cultures of human hepatocytes.

Pharmacokinetics of pradefovir and PMEA in healthy volunteers after oral dosing of pradefovir

The pharmacokinetics of pradefovir and adefovir, 9-(2-phosphonylmethoxyethyl) adenine (PMEA), was evaluated in healthy male volunteers after oral dosing of pradefovir (10, 30, or 60 mg). Pradefovir was absorbed rapidly. The maximum serum concentration, the area under the concentration-time curve between 0 and 96 hours after dosing (AUC(0-96)), and the area under the plasma concentration versus time curve from time 0 to infinity (AUC(0-infinity)) of pradefovir and PMEA increased with the dose of pradefovir. The ratio of PMEA to pradefovir for AUC(0-96) and AUC(0-infinity) ranged from 1.4 to 1.8. Renal clearance of pradefovir (18-31 L/h) increased with the dose of pradefovir and was greater than glomerular filtration. The fraction of total body clearance due to renal clearance was low (0.045 to 0.083), suggesting that metabolic clearance played a significant role in the clearance of pradefovir in man. In addition, an evaluation of the food effect was conducted at the 30-mg dose. The results indicate that food intake has no effect on the extent of exposure of pradefovir and PMEA but may decrease the rate of systemic availability of PMEA.

Single-dose pharmacokinetics and metabolism of [14C]remofovir in rats and cynomolgus monkeys

Single-dose pharmacokinetics and metabolism of [(14)C]remofovir was studied in rats and monkeys following intravenous (i.v.) and oral administration (30 mg/kg of body weight). Oral absorption and bioavailability were 29.7 and 5.42% in rats and 65.6 and 19.4% in monkeys, respectively. Following i.v. administration, the elimination half-life for remofovir was 0.7 h in both rats and monkeys. Total body clearance was 5.85 liters/h/kg in rats and 2.60 liters/h/kg in monkeys; apparent volume of distribution was 5.99 liters/kg in rats and 2.70 liters/kg in monkeys. Following oral administration, remofovir was extensively converted to 9-(2-phosphonylmethoxyethyl)adenine (PMEA) and other metabolites in both species. In rats, excretion of total radioactivity in urine accounted for 61.8% of the i.v. dose and 12.9% of the oral dose, while in monkeys it accounted for 43.3% of the i.v. dose and 34.9% of the oral dose. Following i.v. dosing of [(14)C]remofovir, fecal excretion of radioactivity accounted for 37.5% of the dose in rats and 17.4% of the dose in monkeys, indicating significant biliary excretion of the drug in animals. PMEA and metabolite A were the major urinary metabolites in both species after i.v. and oral administration of remofovir.