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

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.

New developments in antiviral therapy for chronic hepatitis B

Chronic hepatitis B affects approximately 400 million people in the world with a substantial disease burden like liver cirrhosis and hepatocellular carcinoma (HCC). Treatment for chronic hepatitis B has improved dramatically in the last decade, resulting in more patients achieving a state of inactive disease. Currently two treatment strategies are available; treatment with peginterferon (peg-IFN) or nucleos(t)ide analogues with the aim to suppress hepatitis B virus (HBV) DNA to subsequently avoid the development of cirrhosis and HCC. Unfortunately, treatment with peg-IFN can be suboptimal with important adverse effects and nucleos(t)ide analogues provoke resistance. At present, no new promising compounds attacking the HBV life cycle are in development. However, for prediction of sustained response or treatment failure, data from the long-term large peg-IFN trials provide important response markers. For the future the focus is to achieve HBsAg loss and anti-HBs conversion which is the closest the treatment can get to a cure. This review summarizes the current treatment options with their response rates and discusses future strategies for chronic hepatitis B treatment.

Synthesis and biological evaluation of a series of liver-selective phosphonic acid thyroid hormone receptor agonists and their prodrugs

Phosphonic acid (PA) thyroid hormone receptor (TR) agonists were synthesized to exploit the poor distribution of PA-based drugs to extrahepatic tissues and thereby to improve the therapeutic index. Nine PAs showed excellent TR binding affinities (TRbeta(1), K(i) < 10 nM), and most of them demonstrated significant cholesterol lowering effects in a cholesterol-fed rat (CFR) model. Unlike the corresponding carboxylic acid analogue and T(3), PA 22c demonstrated liver-selective effects by inducing maximal mitochondrial glycerol-3-phosphate dehydrogenase activity in rat liver while having no effect in the heart. Because of the low oral bioavailability of PA 22c, a series of prodrugs was synthesized and screened for oral efficacy in the CFR assay. The liver-activated cyclic 1-(3-chlorophenyl)-1,3-propanyl prodrug (MB07811) showed potent lipid lowering activity in the CFR (ED(50) 0.4 mg/kg, po) and good oral bioavailability (40%, rat) and was selected for development for the treatment of hypercholesterolemia.

Novel therapies in hepatitis B and C

Chronic hepatitis B and C affect approximately 500 million people in the world, with substantial disease burden including liver cirrhosis and hepatocellular carcinoma. For chronic hepatitis B, two treatment strategies are currently available, both with suboptimal response and significant side effects. Promising new drugs are approaching the stage of approval; however, these agents still need further development to control this disease. Based on the understanding of the hepatitis C virus life cycle, new treatment developments for chronic hepatitis C tend to succeed rapidly; therefore, it is only a matter of time before new therapies emerge. This review summarizes the most important new agents available for treatment of chronic hepatitis B and C.

Preclinical pharmacokinetics of a HepDirect prodrug of a novel phosphonate-containing thyroid hormone receptor agonist

The prodrug [(2R,4S)-4-(3-chlorophenyl)-2-[(3,5-dimethyl-4-(4'-hydroxy-3'-isopropylbenzyl)phenoxy)methyl]-2-oxido-[1,3,2]-dioxaphosphonane (MB07811)] of a novel phosphonate-containing thyroid hormone receptor agonist [3,5-dimethyl-4-(4'-hydroxy-3'-isopropylbenzyl)phenoxylmethylphosphonic acid (MB07344)] is the first application of the HepDirect liver-targeting approach to a non-nucleotide agent. The disposition of MB07811 was characterized in rat, dog, and monkey to assess its liver specificity, which is essential in limiting the extrahepatic side effects associated with this class of lipid-lowering agents. MB07811 was converted to MB07344 in liver microsomes from all species tested (CL(int) 1.23-145.4 microl/min/mg). The plasma clearance and volume of distribution of MB07811 matched or exceeded 1 l/h/kg and 3 l/kg, respectively. Although absorption of prodrug was good, its absolute oral bioavailability as measured systemically was low (3-10%), an indication of an extensive hepatic first-pass effect. This effect was confirmed by comparison of systemic exposure levels of MB07811 after portal and jugular vein administration to rats, which demonstrated a hepatic extraction ratio of >0.6 with liver CYP3A-mediated conversion to MB07344 being a major component. The main route of elimination of MB07811 and MB07344 was biliary, with no evidence for enterohepatic recirculation of MB07344. Similar metabolic profiles of MB07811 were obtained in liver microsomes across the species tested. Tissue distribution and whole body autoradiography confirmed that the liver is the major target organ of MB07811 and that conversion to MB07344 was high in the liver relative to that in other tissues. Hepatic first-pass extraction and metabolism of MB07811, coupled with possible selective distribution of MB07811-derived MB07344, led to a high degree of liver targeting of MB07344.

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.

Telbivudine: a hepatitis B-specific antiviral

Telbivudine, a l-nucleoside enantiomer, is a potent specific inhibitor of hepatitis B virus (HBV) polymerase. It recently completed Phase III trials and has been licensed in most countries worldwide. It has shown superior efficacy compared with lamivudine in therapeutic response, reduction in HBV DNA, proportion of patients with undetectable HBV DNA, reduced primary treatment failure and reduced viral resistance over 2 years, in Hepatitis B e antigen-positive and -negative patients. Further studies show that switching to telbivudine in lamivudine-suboptimal responders improves the reduction in HBV DNA at week 24 and shows superiority compared with adefovir in reduction of HBV DNA at week 24. Multivariate analysis showed that week 24 HBV DNA best predicted outcomes at 2 years and a 'hepatitis B roadmap' concept has been proposed to manage patients based on this. Thus, telbivudine is a useful addition to the current landscape of hepatitis B therapeutic agents.

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.

Emerging drugs for hepatitis B

Chronic hepatitis B remains a treatment challenge despite the availability of new nucleoside analogs. This is due to the persistence of viral infection during therapy, which exposes the patient to the risk of developing antiviral drug resistance. Therefore, new polymerase inhibitors are needed to manage resistance to existing drugs and new trials of combination therapy are required to delay drug resistance. In the future, antiviral agents targeting other steps of the viral life cycle will be needed to achieve antiviral synergy and prevent antiviral drug resistance. Immune modulators are also expected to enhance antiviral response and to achieve sustained response. Discovery of new antiviral drugs and design of new treatment strategies are, therefore, needed to manage this disease, which is still the main cause of cirrhosis and hepatocellular carcinoma worldwide.

Antiviral therapy and resistance with hepatitis B virus infection

Hepatitis B virus (HBV) infection is still the most common cause of hepatocellular carcinoma and liver cirrhosis world wide. Recently, however, there has been quite dramatic improvement in the understanding of HBV associated liver disease and its treatment. It has become clear that high viral replication is a major risk factor for the development of both cirrhosis and hepatocellular carcinoma. Early studies have shown lamivudine lowers the risk of HBV associated complications. There are currently three nucleos(t)ides licensed, in addition to interferon, and there are more drugs coming to the market soon. Interferon or its pegylated counterpart are still the only options for treatment with defined end points, while nucleos(t)ides therapy is used mostly for long term treatment. Combination therapies have not been shown to be superior to monotherapy in naïve patients, however, the outcome depends on how the end point is defined. Interferon plus lamivudine achieves a higher viral suppression than either treatment alone, even though Hbe-seroconversion was not different after a one year treatment. HBV-genotypes emerge as relevant factors, with genotypes "A" and "B" responding relatively well to interferon, achieving up to 20% HBsAg clearance in the case of genotype "A". In addition to having a defined treatment duration, interferon has the advantage of lacking resistance selection, which is a major drawback for lamivudine and the other nucleos(t)ides. The emergence of resistance against adefovir and entecavir is somewhat slower in naïve compared to lamivudine resistant patients. Adefovir has a low resistance profile with 3%, 9%, 18%, and 28% after 2, 3, 4, and 5 years, respectively, while entecavir has rarely produced resistance in naïve patients for up to 3 years.

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.

Where does adefovir stand amongst newly developed antivirals: from pharmacology to virology

Adefovir, an acyclic nucleotide analog of adenosine monophosphate, has demonstrated potent activities against several viruses in vitro. Since 2002, the diester prodrug of this molecule, adefovir dipivoxil, has been approved as a well-tolerated and efficacious treatment for chronic hepatitis B. It is the first nucleotide analog to be approved for this indication and it is now challenged by several other molecules that also inhibit hepatitis B virus replication. In this article, we analyze the strengths and limitations of adefovir in the context of hepatitis B treatment and consider the other alternatives in the treatment of this difficult-to-treat chronic disease. Adefovir dipivoxil has offered a real opportunity for patients who developed lamivudine resistance and were facing a therapeutic deadlock; the next challenge for clinicians will be to offer the best therapeutic strategy to limit the inexorable selection of resistant strains.