In vitro and in vivo pharmacokinetic characterization of two novel prodrugs of zidovudine

Understanding the pharmacokinetics of prodrug and metabolite

This tutorial explains the pharmacokinetics of a prodrug and its active metabolite (or parent drug) using a two-step, consecutive, first-order irreversible reaction as a basic model for prodrug metabolism. In this model, the prodrug is metabolized and produces the parent drug, which is subsequently eliminated. The mathematical expressions for pharmacokinetic parameters were derived step by step. In addition, we visualized these expressions to help understand the relationship between pharmacokinetic parameters easily. For the elimination rate-limited and formation rate-limited metabolism, we analyzed the plasma drug concentration versus time curve of a prodrug administered intravenously.

Acute liver failure enhances oral plasma exposure of zidovudine in rats by downregulation of hepatic UGT2B7 and intestinal P-gp

HIV infection is often associated with liver failure, which alters the pharmacokinetics of many drugs. In this study we investigated whether acute liver failure (ALF) altered the pharmacokinetics of the first-line anti-HIV agent zidovudine (AZT), a P-gp/BCRP substrate, in rats. ALF was induced in rats by injecting thioacetamide (TAA, 300 mg·kg^-1·d^-1, ip) for 2 days. On the second day after the last injection of TAA, the pharmacokinetics of AZT was investigated following both oral (20 mg/kg) and intravenous (10 mg/kg) administration. ALF significantly increased the plasma concentrations of AZT after both oral and intravenous doses of AZT, but without affecting the urinary excretion of AZT. AZT metabolism was studied in rat hepatic microsomes in vitro, which revealed that hepatic UGT2B7 was the main enzyme responsible for the formation of AZT O-glucuronide (GAZT); ALF markedly impaired AZT metabolism in hepatic microsomes, which was associated with the significantly decreased hepatic UGT2B7 expression. Intestinal absorption of AZT was further studied in rats via in situ single-pass intestinal perfusion. Intestinal P-gp function and intestinal integrity were assessed with rhodamine 123 and FD-70, respectively. We found that ALF significantly downregulated intestinal P-gp expression, and had a smaller effect on intestinal BCRP. Further studies showed that ALF significantly increased the intestinal absorption of both rhodamine 123 and AZT without altering intestinal integrity, thus confirming an impairment of intestinal P-gp function. In conclusion, ALF significantly increases the oral plasma exposure of AZT in rats, a result partly attributed to the impaired function and expression of hepatic UGT2B7 and intestinal P-gp.

Single-Pass Intestinal Perfusion (SPIP) and prediction of fraction absorbed and permeability in humans: A study with antiretroviral drugs

In recent years, the prediction of oral drug absorption in humans has been a challenge for researchers and many techniques for permeability studies have been developed for several purposes, including biowaiver processes. The Single-Pass Intestinal Perfusion (SPIP) method performed in rats can provide permeability results closest to in vivo condition. The purpose of the present study was to evaluate the intestinal permeability of the antiretroviral drugs lamivudine, stavudine and zidovudine using the SPIP method in rats and to predict their permeability (Peff,humans) and fraction absorbed (Fa) in humans. Metoprolol and fluorescein were used as marker compounds of high and low permeability, respectively. The effective permeability (Peff) results showed that stavudine and zidovudine have high permeability characteristics while lamivudine presented the lowest result. From Peff values obtained in rats, the Peff,humans and Fa were calculated. The use of SPIP in rats and calculations for absorption prediction in humans may indicate the transport mechanisms and/or pre-systemic metabolism involved on permeation processes of drugs, since this model is the closest to in vivo conditions.

Warfarin is an effective modifier of multiple UDP-glucuronosyltransferase enzymes: evaluation of its potential to alter the pharmacokinetics of zidovudine

In this study, we aimed to determine the modulatory effects of warfarin (an extensively used anticoagulant drug) and its metabolites on UDP-glucuronosyltransferase (UGT) activity and to assess the potential of warfarin to alter the pharmacokinetics of zidovudine (AZT). The effects of warfarin and its metabolites on glucuronidation were determined using human and rat liver microsomes (HLM and RLM) as well as expressed UGTs. The mechanisms of warfarin-UGT interactions were explored through kinetic characterization and modeling. Pharmacokinetic studies with rats were performed to evaluate the potential of warfarin to alter the pharmacokinetics of AZT. We found that warfarin was an effective modifier of a panel of UGT enzymes. The effects of warfarin on glucuronidation were inhibitory for UGT1A1, 2B7, and 2B17, but activating for UGT1A3. Mixed effects were observed for UGT1A7 and 1A9. Consistent with its inhibitory effects on UGT2B7 activity, warfarin inhibited AZT glucuronidation in HLM (Ki = 74.9-96.3 μM) and RLM (Ki = 190-230 μM). Inhibition of AZT glucuronidation by UGT2B7, HLM, and RLM was also observed with several hydroxylated metabolites of warfarin. Moreover, the systemic exposure (AUC) of AZT in rats was increased by a 1.5- to 2.1-fold upon warfarin coadministration. The elevated AUC was associated with suppressed glucuronidation that was probably attained through a combined action of warfarin and its hydroxylated metabolites. In conclusion, the activities of multiple UGT enzymes can be modulated by warfarin and the nature of modulation was isoform dependent. Also, pharmacokinetic interactions of zidovudine with warfarin were highly possible through inhibition of UGT metabolism.

Intestinal permeability of lamivudine (3TC) and two novel 3TC prodrugs. Experimental and theoretical analyses

Lamivudine (3TC) is an antiviral drug with a widely demonstrated clinical efficacy used to treat Acquired Immunodeficiency Syndrome (AIDS) in humans. However, since the rapid emergence of resistant viral strains has limited its effect, several new strategies such as the design of prodrugs have been applied to try to optimize its pharmacotherapeutic properties. The present study deals with the intestinal permeation of 3TC and two novel prodrugs of 3TC, namely 3TC-Etha and 3TC-Buta, by using rat intestinal segments and applying the gut sac in vitro technique. An adequate bioanalytical method (sample preparation and quantitative analysis) was fully developed and validated for the quantitation of these three compounds. A low permeability coefficient (P(app) 0.408 ± 0.049 × 10(-4) cm/min) was found for 3TC if compared to that previously reported for zidovudine (2.38 ± 0.12 × 10(-4) cm/min), while no statistically significant differences were observed in its apical-to-basal or basal-to-apical permeabilities. Our data suggests that 3TC permeates through the intestinal tissue by passive diffusion, with no intervention of efflux mechanism during this process as determined applying the gut sac technique. Regarding the prodrugs, both 3TC-Etha and 3TC-Buta were able to increase 3TC permeability (2 and 10 times, respectively), but none of these compounds were capable of crossing the intestinal tissue in their intact forms. In the case of 3TC-Etha, the permeability of the intact compound (P(app) 0.093 ± 0.010 × 10(-4) cm/min) was impaired by a P-glycoprotein (P-gp) mediated efflux, evidenced by the higher permeability coefficient (6.933 ± 0.586 × 10(-4) cm/min) determined in the presence of verapamil on the apical side of the enterocyte. In contrast, 3TC-Buta was not subjected to efflux mechanisms on the apical side of the enterocyte, but was efficiently converted to 3TC by enzymatic hydrolysis during the permeation process. In the light of these results, molecular modeling (docking and molecular dynamics) techniques were applied to study further the structural features that may confer the different behaviors of these two compounds with respect to P-gp mediated efflux. The results also highlight the potential of combining experimental and theoretical studies in the design of 3TC prodrugs with enhanced intestinal permeation properties.

Engineered human Tmpk fused with truncated cell-surface markers: versatile cell-fate control safety cassettes

Cell-fate control gene therapy (CFCGT)-based strategies can augment existing gene therapy and cell transplantation approaches by providing a safety element in the event of deleterious outcomes. Previously, we described a novel enzyme/prodrug combination for CFCGT. Here, we present results employing novel lentiviral constructs harboring sequences for truncated surface molecules (CD19 or low-affinity nerve growth factor receptor) directly fused to that CFCGT cDNA (TmpkF105Y). This confers an enforced one-to-one correlation between cell marking and eradication functions. In-vitro analysis demonstrated the full functionality of the fusion product. Next, low-dose 3'-azido-3'-deoxythymidine (AZT) administration to non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice injected with transduced clonal K562 cells suppressed tumor growth; furthermore, one integrated vector on average was sufficient to mediate cytotoxicity. Further, in a murine xenogeneic leukemia-lymphoma model we also demonstrated in-vivo control over transduced Raji cells. Finally, in a proof-of-principle study to examine the utility of this cassette in combination with a therapeutic cDNA, we integrated this novel CFCGT fusion construct into a lentivector designed for treatment of Fabry disease. Transduction with this vector restored enzyme activity in Fabry cells and retained AZT sensitivity. In addition, human Fabry patient CD34(+) cells showed high transduction efficiencies and retained normal colony-generating capacity when compared with the non-transduced controls. These collective results demonstrated that this novel and broadly applicable fusion system may enhance general safety in gene- and cell-based therapies.

Everted gut sac model as a tool in pharmaceutical research: limitations and applications

OBJECTIVES

This review discusses the limitations and applications of the everted gut sac model in studying drug absorption, metabolism, and interaction.

KEY FINDINGS

The mechanism of drug absorption, interaction and the effect of factors such as age, sex, species, chronic therapy, and disease state on drug absorption have been summarized. The experimental conditions and their effects on the outcomes of trials have been discussed also.

SUMMARY

The everted sac model is an efficient tool for studying in-vitro drug absorption mechanisms, intestinal metabolism of drugs, role of transporter in drug absorption, and for investigating the role of intestinal enzymes during drug transport through the intestine.

Poly(HEMA-Zidovudine) conjugate: a macromolecular pro-drug for improvement in the biopharmaceutical properties of the drug

A macromolecular pro-drug of a known anti-viral agent Zidovudine (AZT) was synthesized and evaluated as a sustained drug delivery system. The pro-drug was synthesized by coupling the drug to 2-hydroxyethyl methacrylate (HEMA) through a succinic spacer to get a monomeric drug conjugate which was polymerized to obtain the polymeric pro-drug. The pro-drug was subjected for in-vitro drug release study in buffers of pH 1.2 and 7.4. The hydrolytic stability of the pro-drug to pepsin was assessed in simulated gastric fluid (SGF, pH 1.2) and to α-chymotrypsin in simulated intestinal fluid (SIF, pH 7.4). The results showed that the drug release from the polymeric backbone takes place in a sustained manner over a period of 24 h, and the amount of drug released was comparatively higher at pH 7.4. Plasmatic hydrolysis studies of succinylzidovudine showed nearly complete release of AZT. At all pH conditions in the presence and absence of α-chymotrypsin, AZT was released preferentially in comparison with the succinyl derivative. The in-vivo release studies in rabbits after oral administration of AZT conjugate demonstrated a sustained release of parent drug over a period of 24 h. The pro-drug provided a significant increase in the area under the plasma concentration time curve as compared to free drug and extended the plasma half-life from 1.06 h to 8.08 h. This study suggested that, after oral administration, the drug-polymer conjugate can release AZT for prolonged periods, thus improving the pharmacokinetics of AZT and decreasing the fluctuation in plasma drug levels that can lead to toxicity.