Relationship between plasma concentrations of 3'-deoxy-3'-fluorothymidine (alovudine) and antiretroviral activity in two concentration-controlled trials

Repurposing nucleoside reverse transcriptase inhibitors (NRTIs) to slow aging

Repurposing drugs already approved in the clinic to be used off-label as geroprotectors, compounds that combat mechanisms of aging, are a promising way to rapidly reduce age-related disease incidence in society. Several recent studies have found that a class of drugs-nucleoside reverse transcriptase inhibitors (NRTIs)-originally developed as treatments for cancers and human immunodeficiency virus (HIV) infection, could be repurposed to slow the aging process. Interestingly, these studies propose complementary mechanisms that target multiple hallmarks of aging. At the molecular level, NRTIs repress LINE-1 elements, reducing DNA damage, benefiting the hallmark of aging of 'Genomic Instability'. At the organellar level, NRTIs inhibit mitochondrial translation, activate ATF-4, suppress cytosolic translation, and extend lifespan in worms in a manner related to the 'Loss of Proteostasis' hallmark of aging. Meanwhile, at the cellular level, NRTIs inhibit the P2X7-mediated activation of the inflammasome, reducing inflammation and improving the hallmark of aging of 'Altered Intercellular Communication'. Future development of NRTIs for human aging health will need to balance out toxic side effects with the beneficial effects, which may occur in part through hormesis.

The thymidine dideoxynucleoside analog, alovudine, inhibits the mitochondrial DNA polymerase γ, impairs oxidative phosphorylation and promotes monocytic differentiation in acute myeloid leukemia

Mitochondrial DNA encodes 13 proteins that comprise components of the respiratory chain that maintain oxidative phosphorylation. The replication of mitochondrial DNA is performed by the sole mitochondrial DNA polymerase γ. As acute myeloid leukemia (AML) cells and stem cells have an increased reliance on oxidative phosphorylation, we sought to evaluate polymerase γ inhibitors in AML. The thymidine dideoxynucleoside analog, alovudine, is an inhibitor of polymerase γ. In AML cells, alovudine depleted mitochondrial DNA, reduced mitochondrial encoded proteins, decreased basal oxygen consumption, and decreased cell proliferation and viability. To evaluate the effects of polymerase γ inhibition with alovudine in vivo, mice were xenografted with OCI-AML2 cells and then treated with alovudine. Systemic administration of alovudine reduced leukemic growth without evidence of toxicity and decreased levels of mitochondrial DNA in the leukemic cells. We also showed that alovudine increased the monocytic differentiation of AML cells. Genetic knockdown and other chemical inhibitors of polymerase γ also promoted AML differentiation, but the effects on AML differentiation were independent of reductions in oxidative phosphorylation or respiratory chain proteins. Thus, we have identified a novel mechanism by which mitochondria regulate AML fate and differentiation independent of oxidative phosphorylation. Moreover, we highlight polymerase γ inhibitors, such as alovudine, as novel therapeutic agents for AML.

Imaging peritoneal metastasis of gastric cancer with 18F-fluorothymidine positron emission tomography/computed tomography: a proof-of-concept study

OBJECTIVE

Peritoneal metastasis (PM) is the most frequent form of metastasis in gastric cancer (GC). The sensitivity of detecting PM by pre-operative imaging modalities is low. Utility of positron emission tomography (PET) with ¹⁸F-fluodeoxyglucose (FDG) for GC is limited, because diffuse-type tumors are not FDG-avid. ¹⁸F-fluothymidine ([F-18]FLT) is a radiotracer that reflects cellular proliferation and the utility of [F-18]FLT-PET in GC has been reported. In this proof-of-concept study, we explored the ability of [F-18]FLT-PET/CT to detect PM of GC previously identified by other imaging modalities.

METHODS

The key eligibility criteria were as follows; (i) histologically proven gastric adenocarcinoma; (ii) evident PM detected by CT performed within 4 weeks prior to registration; (iii) no prior treatment of PM within 4 weeks before registration. [F-18]FLT-PET/CT was performed at National Cancer Center Hospital, and [F-18]FLT-PET/CT images were evaluated independently by two radiologists. Safety assessments were carried out before and after [F-18]FLT-PET/CT. The primary end point was the detection sensitivity of PM.

RESULTS

A total of 19 eligible patients were analyzed, of which 15 (78.9%) had diffuse-type histology. Detection sensitivity of PM, primary lesion, and lymph node metastasis were 73.7% [maximum standardized uptake value (SUVmax): 1.697-13.21], 100% (SUVmax: 2.71-22.01), and 72.7% (SUVmax: 2.079-12.61), respectively. No patients experienced adverse events during or after [F-18]FLT-PET/CT.

CONCLUSION

This proof-of-concept study shows that [F-18]FLT-PET/CT is a sensitive method for detecting PM in GC, and paves the way for future studies investigating the clinical utility of this approach for the detection of clinically non-evident PM in GC. Advances in knowledge: This proof-of-concept study found that [F-18]FLT-PET/CT is a sensitive method for detecting peritoneal metastases in GC.

Comparison of characteristics of 18F-fluorodeoxyglucose and 18F-fluorothymidine PET during staging of esophageal squamous cell carcinoma

OBJECTIVES

The aim of this study was to compare the characteristics derived from fluorine-18 fluorodeoxyglucose (F-FDG) and fluorine-18 fluorothymidine (F-FLT) PET quantitatively and to assess their capacities during staging of esophageal squamous cell carcinoma (ESCC).

MATERIALS AND METHODS

Thirty-six patients with a diagnosis of ESCC who underwent both F-FDG and F-FLT PET were included in this study. Different image-derived indices including the standardized uptake value (SUV), gross tumor length, and texture features were determined. Considering histopathologic examination as the gold standard, the performance of the extracted indices during staging of ESCC was assessed using the Kruskal-Wallis test and the Mann-Whitney test.

RESULTS

Considering the F-FDG PET images, the SUVmax, SUVmean, length (LEN), and eccentricity (EC) were significant during staging of American Joint Committee on Cancer (AJCC) and TNM (P<0.01), whereas for the F-FLT image, the SUVmax, LEN, and EC were significant during staging of AJCC and TNM (P<0.01). The characteristics of F-FDG and F-FLT PET for the classification of ESCC stage were significantly different.

CONCLUSION

F-FDG image-derived characteristics including image textural features, SUV, and shape feature allow for better stratification of AJCC and TNM than F-FLT PET in ESCC patients.

Distribution to the Brain and Protein Binding of 3′and 5-Substituted 2′,3′-Dideoxyuridine Derivatives, Studied by Microdialysis

The aim of this study was to investigate a series of 3′ and 5-substituted 2′,3′-dideoxyuridine derivatives (ddUD) with respect to plasma protein binding, half-life and distribution across the blood-brain barrier in the rat. The microdialysis technique was used to study protein binding in human plasma ( in vitro), and to sample the extracellular space of rats with microdialysis probes implanted into the striatum of the brain and the gastrocnemic muscle ( in vivo). The compounds were analysed by HPLC with UV-detection. The octanol/water partition coefficients of the ddUD varied from 0.08-0.84. The protein binding of the ddUDs was approximately 80%. After s.c. administration (25 or 50 mg kg−1), the brain and muscle extracellular levels differed; brain levels were 0.18-0.36 of peripheral (muscle) concentrations. A multivariate analysis, which included data on zidovudine, alovudine and thymidine, demonstrated a relationship between the physicochemical and some of the pharmacokinetic properties of uridine analogues. The analysis shows that half-life and protein binding increases with decreasing p Ka. However, penetration to the brain is not correlated with the partition into octanol. It is concluded that the transport to the brain is not primarily dependent upon passive diffusion over a lipophilic barrier but, rather, to other chemical properties of the ddUDs. This is suggestive of a specific transport mechanism, e.g. the thymidine carrier.

18F-FLT PET imaging of cellular proliferation in pancreatic cancer

Pancreatic ductal adenocarcinoma is known for its poor prognosis. Since the development of computerized tomography, magnetic resonance and endoscopic ultrasound, novel imaging techniques have struggled to get established in the management of patients diagnosed with pancreatic adenocarcinoma for several reasons. Thus, imaging assessment of pancreatic cancer remains a field with scope for further improvement. In contrast to cross-sectional anatomical imaging methods, molecular imaging modalities such as positron emission tomography (PET) can provide information on tumour function. Particularly, tumour proliferation may be assessed by measurement of intracellular thymidine kinase 1 (TK1) activity level using thymidine analogues radiolabelled with a positron emitter for use with PET. This approach, has been widely explored with [(18)F]-fluoro-3'-deoxy-3'-L-fluorothymidine ((18)F-FLT) PET. This manuscript reviews the rationale and physiology behind (18)F-FLT PET imaging, with special focus on pancreatic cancer and other gastrointestinal malignancies. Potential benefit and challenges of this imaging technique for diagnosis, staging and assessment of treatment response in abdominal malignancies are discussed.

FDG and FLT-PET for Early measurement of response to 37.5 mg daily sunitinib therapy in metastatic renal cell carcinoma

BACKGROUND

Metastatic renal cell carcinoma has a poor prognosis and an intrinsic resistance to standard treatment. Sunitinib is an oral receptor tyrosine kinase inhibitor that has been used as a first-line targeted therapy in metastatic renal cell carcinoma. While computed tomography (CT) is currently the gold standard for response assessment in oncological trials, numerous studies have shown that positron emission tomography (PET) imaging can provide information predictive of tumor response to treatment earlier than the typical interval for standard of care follow-up CT imaging. In this exploratory study we sought to characterize early tumor response in patients with metastatic renal cell carcinoma treated with continuous daily 37.5 mg sunitinib therapy.

METHODS

Twenty patients underwent dynamic acquisition positron emission tomography (PET) imaging using (18) F-fluorodeoxyglucose (FDG) and (18) F-fluorothymidine (FLT) at baseline and early in treatment (after 1, 2, 3 or 4 weeks) with 37.5 mg continuous daily dosing of sunitinib. Semi-quantitative analyses were performed to characterize the tumor metabolic (FDG) and proliferative (FLT) responses to treatment.

RESULTS

Proliferative responses were observed in 9/19 patients and occurred in 2 patients at one week (the earliest interval evaluated) after the initiation of therapy. A metabolic response was observed in 5/19 patients, however this was not observed until after two weeks of therapy were completed. Metabolic progression was observed in 2/19 patients and proliferative progression was observed in 1/19 patients. Baseline FDG-PET tumor maximum standardized uptake values correlated inversely with overall survival (p = 0.0036). Conversely, baseline (18) F-fluorothymidine PET imaging did not have prognostic value (p = 0.56) but showed a greater early response rate at 1-2 weeks after initiating therapy.

CONCLUSIONS

While preliminary in nature, these results show an immediate and sustained proliferative response followed by a delayed metabolic response beginning after two weeks in metastatic renal cell carcinoma treated with a continuous daily dose of 37.5 mg sunitinib. The results provide evidence of tumor response to lower-dose sunitinib while also supporting the inclusion of PET imaging as a tool for early assessment in oncological clinical trials.

TRIAL REGISTRATION

ID: NCT00694096.

Pharmacokinetic and Pharmacodynamic Analysis of Amprenavir-Containing Combination Therapy in HIV-1-Infected Children

Several factors influence the antiviral response to antiretroviral therapy. In this pharmacokinetic and pharmacodynamic analysis, the relationship of drug exposure, demographics, and cotherapy measures to antiviral response in a cohort of largely treatment-experienced children treated with amprenavir and nucleoside reverse transcriptase inhibitors was examined. Multiple pharmacodynamic and demographic factors were examined, but only the minimum plasma concentration (C(min))/protein-binding-adjusted 50% inhibitory drug concentration (IC(50)) ratio and whether individuals received 2 versus fewer than 2 nucleosides to which their viral isolates were susceptible were associated with the magnitude of the time-weighted average change in HIV-1 RNA log(10) copies/mL from baseline (AAUCMB). In multivariate logistic regression analysis, only the C(min)/IC(50) ratio was independently associated with having a >or=1 log(10) AAUCMB decline. The probability in the study population of having a >or=1log(10) AAUCMB was 50% and 85% at C(min)/IC(50) ratios of approximately 1 and 4, respectively. Of the multiple factors examined, only the C(min)/IC(50) ratio was a significant predictor of antiviral response in the first 8 weeks on amprenavir-containing combination antiretroviral therapy.

3'-Deoxy-3'-[(18)F]-fluorothymidine ([(18)F]-FLT) transport in newly diagnosed glioma: correlation with nucleoside transporter expression, vascularization, and blood-brain barrier permeability

3'-Deoxy-3'-[(18)F]-fluorothymidine ([(18)F]-FLT), a marker of cellular proliferation, has been used in positron emission tomography (PET) examination of gliomas. The aim of this study was to investigate whether the uptake of [(18)F]-FLT in glioma correlates with messenger RNA (mRNA) levels of the equilibrative nucleoside transporter 1 (ENT1), microvascular density (assessed by CD34 immunohistochemistry), and the blood-brain barrier (BBB) breakdown. A total of 21 patients with newly diagnosed glioma were examined with [(18)F]-FLT PET. Tumor lesions were identified as areas of focally increased [(18)F]-FLT uptake, exceeding that of surrounding normal tissue. Dynamic analysis of [(18)F]-FLT PET revealed correlations between the phosphorylation rate constant k 3 and ENT1 expression; however there was no correlation between the kinetic parameters and CD34 score. There was a good correlation between the gadolinium (Gd) enhancement score (evaluating BBB breakdown) and ENT1 expression, CD34 score, and Ki-67 index. This preliminary study suggests that ENT1 expression might not reflect accumulation of [(18)F]-FLT in vivo due to BBB permeability in glioma.

Balancing antiviral potency and host toxicity: identifying a nucleotide inhibitor with an optimal kinetic phenotype for HIV-1 reverse transcriptase

Two novel thymidine analogs, 3'-fluoro-3'-deoxythymidine (FLT) and 2',3'-didehydro-3'-deoxy-4'-ethynylthymidine (Ed4T), have been investigated as nucleoside reverse transcriptase inhibitors (NRTIs) for treatment of HIV infection. Ed4T seems very promising in phase II clinical trials, whereas toxicity halted FLT development during this phase. To understand these different molecular mechanisms of toxicity, pre-steady-state kinetic studies were used to examine the interactions of FLT and Ed4T with wild-type (WT) human mitochondrial DNA polymerase γ (pol γ), which is often associated with NRTI toxicity, as well as the viral target protein, WT HIV-1 reverse transcriptase (RT). We report that Ed4T-triphosphate (TP) is the first analog to be preferred over native nucleotides by RT but to experience negligible incorporation by WT pol γ, with an ideal balance between high antiretroviral efficacy and minimal host toxicity. WT pol γ could discriminate Ed4T-TP from dTTP 12,000-fold better than RT, with only an 8.3-fold difference in discrimination being seen for FLT-TP. A structurally related NRTI, 2',3'-didehydro-2',3'-dideoxythymidine, is the only other analog favored by RT over native nucleotides, but it exhibits only a 13-fold difference (compared with 12,000-fold for Ed4T) in discrimination between the two enzymes. We propose that the 4'-ethynyl group of Ed4T serves as an enzyme selectivity moiety, critical for discernment between RT and WT pol γ. We also show that the pol γ mutation R964C, which predisposes patients to mitochondrial toxicity when receiving 2',3'-didehydro-2',3'-dideoxythymidine to treat HIV, produced some loss of discrimination for FLT-TP and Ed4T-TP. These molecular mechanisms of analog incorporation, which are critical for understanding pol γ-related toxicity, shed light on the unique toxicity profiles observed during clinical trials.

Principles and Application of PET in Brain Tumors

For tumors of the central nervous system (CNS), the ability to accurately delineate the extent of tumor has implications for diagnosis, prognosis, and treatment. PET, mainly with (18)F-fluorodeoxyglucose (FDG), has become commonplace in the work-up of many extracranial tumors. However, the relative high background of FDG-PET activity of normal brain tissue has limited the applicability of this modality in CNS tumors to date. More recently, novel PET tracers for imaging of CNS tumors have been developed. This article outlines recent advances in PET as a complementary imaging modality with implications for diagnosis, prognosis, surgical and radiation treatment planning, and post-therapy surveillance in malignancies of the CNS. Pharmacokinetic properties of the radiotracers and the influence of blood-brain-barrier integrity are also incorporated into the discussion.

The kinetic effects on thymidine kinase 2 by enzyme-bound dTTP may explain the mitochondrial side effects of antiviral thymidine analogs

Mitochondrial thymidine kinase 2 (TK2) is a key enzyme in the salvage of pyrimidine deoxynucleosides needed for mitochondrial DNA synthesis. TK2 phosphorylates thymidine (dThd), deoxycytidine (dCyd), and many other antiviral pyrimidine nucleoside analogs. Zidovudine (AZT) is the first nucleoside analog approved for anti-HIV therapy, and it is still used in combination with other drugs. One of the side effects of long-term treatment with nucleoside analogs is mitochondrial DNA depletion, which has been ascribed to competition by AZT for the endogenous dThd phosphorylation carried out by TK2. Here we studied the kinetics of AZT and 3'-fluorothymidine phosphorylation by recombinant human TK2 and the effects of these and other pyrimidine nucleoside analogs on the phosphorylation of dThd and dCyd. Thymidine analogs strongly inhibited dThd phosphorylation but not dCyd phosphorylation, which instead was stimulated ∼30%. We found that recombinant human TK2 contained the feedback inhibitor dTTP in a 1:1 molar ratio and that incubation with dThd and AZT could completely remove the enzyme-bound dTTP, but dCyd was less efficient in this regard. The release of feedback inhibitor by dThd and dThd analogs most likely accounts for the observed kinetics. Similar effects were also observed with native rat liver mitochondrial TK2, strongly indicating a physiologic role for this process, which most likely is an important factor in the mitochondrial toxicity observed with antiviral nucleoside analogs.

Molecular PET and PET/CT imaging of tumour cell proliferation using F-18 fluoro-L-thymidine: a comprehensive evaluation

Positron emission tomography (PET) using F-18 fluoro-3'-deoxy-3-L-fluorothymidine (FLT) offers noninvasive assessment of cell proliferation in vivo. The most important application refers to the evaluation of tumour proliferative activity, representing a key feature of malignancy. Most data to date suggest that FLT is not a suitable biomarker for staging of cancers. This is because of the rather low fraction of tumour cells that undergo replication at a given time with subsequently relatively low tumour FLT uptake. In addition, generally, the high FLT uptake in liver and bone marrow limits the diagnostic use. We describe the current status on preclinical and clinical applications of FLT-PET including our own experience in brain tumours. The future of FLT-PET probably lies in the evaluation of tumour response to therapy and more importantly, in the prediction of early response in the course of treatment. The level of FLT accumulation in tumours depends on thymidine kinase 1 activity and on the therapy-induced activation of the salvage pathway and expression of nucleoside transporters. Therefore, cytostatic agents that cause arrest of the cell cycle in the S-phase may initially increase FLT uptake rather than reducing the tumour cell accumulation. In addition, agents that block the endogenous thymidine pathway may lead to overactivity of the salvage pathway and increase tumour FLT uptake. In contrast, many therapeutic agents inhibit both pathways and subsequently reduce tumour FLT uptake. Further studies comparing FLT with F-18 fluorodeoxyglucose-PET will be important to determine the complementary advantage of FLT-PET in early cancer therapy response assessment. Further research should be facilitated by simplified synthesis of FLT with improved yields and an increasing commercial availability.

Nucleoside and nucleotide HIV reverse transcriptase inhibitors: 25 years after zidovudine

Twenty-five years ago, nucleoside analog 3'-azidothymidine (AZT) was shown to efficiently block the replication of HIV in cell culture. Subsequent studies demonstrated that AZT acts via the selective inhibition of HIV reverse transcriptase (RT) by its triphosphate metabolite. These discoveries have established the first class of antiretroviral agents: nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs). Over the years that followed, NRTIs evolved into the main component of antiretroviral drug combinations that are now used for the treatment of all populations of HIV infected patients. A total of thirteen NRTI drug products are now available for clinical application: eight individual NRTIs, four fixed-dose combinations of two or three NRTIs, and one complete fixed-dose regimen containing two NRTIs and one non-nucleoside RT inhibitor. Multiple NRTIs or their prodrugs are in various stages of clinical development and new potent NRTIs are still being identified through drug discovery efforts. This article will review basic principles of the in vitro and in vivo pharmacology of NRTIs, discuss their clinical use including limitations associated with long-term NRTI therapy, and describe newly identified NRTIs with promising pharmacological profiles highlighting those in the development pipeline. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, volume 85, issue 1, 2010.

Simplified, automated synthesis of 3′[18F]fluoro-3′-deoxy-thymidine ([18F]FLT) and simple method for metabolite analysis in plasma

3′[18F]Fluoro-3′-deoxy-thymidine ([18F]FLT) (III) has been discussed to be a promising tracer for assessing tumor proliferation. In order to perform clinical studies for evaluating [18F]FLT a simplified labeling procedure was developed using 2,3′-anhydrothymidine with benzoyl as a protecting group in the 5′-position (I). In DMSO the labeling yield was 46% at 160 °C in 10 min. Hydrolysis was efficiently performed with 0.25% NaOH at room temperature within 10 min. The labeling procedure was transferred to a remote controlled synthesis module allowing the production of [18F]FLT in high activities. The overall radiochemical yield was 18.1 ± 5.4% (n= 55) with absolute yields of 9.2 ± 2.6 GBq of [18F]FLT at EOS ready for injection (60 min after EOB; irradiation parameters: 35 μA, 60 min) and specific activities of 100–220 GBq/μmol. A convenient cartridge method for metabolite analysis was developed and validatedversusHPLC showing that after 90 min 69.0 ± 7.0% of the radioactivity in plasma (less than 20% of initial radioactivity) was unchanged [18F]FLT (26 patients with various tumors).

Mitochondrial DNA depletion in rat liver induced by fosalvudine tidoxil, a novel nucleoside reverse transcriptase inhibitor prodrug

Fosalvudine tidoxil is a prodrug derived from the nucleoside reverse transcriptase inhibitor 3-deoxy-3-fluorothymidine (FLT; alovudine). FLT effectively inhibits resistant human immunodeficiency virus type 1, but its clinical development was stopped due to bone marrow and liver toxicity. In this study, we examined the long-term in vivo effects of fosalvudine tidoxil on the mitochondrial DNA (mtDNA) contents in rats. Sprague-Dawley rats received fosalvudine tidoxil (15, 40, or 100 mg/kg of body weight/day) by oral gavage during a period of 8 weeks. Didanosine (100 mg/kg/day) was used as a positive control for mitochondrial toxicity. mtDNA levels in liver, gastrocnemius muscle, sciatic nerve, and inguinal fat pad tissues were quantified by real-time PCR. In hepatic mitochondria, fosalvudine tidoxil induced significant mtDNA depletion. At doses of 15, 40, and 100 mg/kg, the mean hepatic mtDNA values were 62, 64, and 47% of control values, respectively. Rats exposed to 100 mg/kg of fosalvudine tidoxil, unlike all other groups, had slightly elevated levels of glutamate pyruvate transaminase in sera. Didanosine induced a loss of mtDNA (to 48% of the control level) similar to that induced by fosalvudine tidoxil. mtDNA levels in skeletal, neural, and adipose tissues in the negative control and treatment groups were similar. Our results suggest that fosalvudine tidoxil induces mitochondrial hepatotoxicity and that this effect warrants scrutiny in clinical trials.

NCI-sponsored trial for the evaluation of safety and preliminary efficacy of FLT as a marker of proliferation in patients with recurrent gliomas: safety studies

PURPOSE

3'-[F-18]Fluoro-3'-deoxythymidine (FLT) is an analog of thymidine that is being developed for imaging cellular proliferation. The goal of this study was to prove that the dose of FLT used for positron emission tomography imaging produces no significant toxicity.

PROCEDURES

Twelve patients with gliomas with either recurrence or suspected radionecrosis were imaged with FLT. Before and at several time points after imaging, subjects underwent general physical and neurological examinations with review of systems and tests of hematologic, hepatic, renal, and several other metabolic parameters. Vital signs and electrocardiograms were monitored during and after the imaging session.

RESULTS

There were no significant adverse effects from FLT injected at a dose of 0.07 mCi/kg (maximum of 5 mCi) at specific activities of 1.25 Ci/micromol or higher. The FLT mass administered for imaging was 0.0001% to 0.0009% of the least toxic cumulative dose administered in clinical trials of FLT as an antiretroviral agent.

CONCLUSIONS

FLT is a safe radiotracer for quantifying proliferation in the human cancer setting.

The evolving role of nuclear molecular imaging in cancer

BACKGROUND: Novel therapies targeted to specific tumor pathways are entering the clinic. The need for in vivo monitoring of resulting molecular changes, particularly with respect to the tumor microenvironment, is growing. Molecular imaging is evolving to include a variety of imaging methods to enable in vivo monitoring of cellular and molecular processes. OBJECTIVES: This article reviews the emerging role of molecular imaging in the development of improved therapeutic strategies that provide better patient selection for therapeutic personalization (i.e. determine which therapies have the greatest chance of success given the individual patient's disease genetic, and phenotypical profile). METHODS: In order to illustrate the utility of integrating molecular imaging into therapy development strategies, current and emerging applications of nuclear molecular imaging strategies will be compared with conventional strategies. Proposed methods of integrating molecular imaging techniques into cancer therapeutic development and limitations of these techniques will be discussed. RESULTS/CONCLUSION: Molecular imaging provides a variety of new tools to accelerate the development of cancer therapies. The recent drive to develop molecular imaging probes and standardize molecular imaging techniques is creating the scaffolding for the evolving paradigm shift to personalized cancer therapy.

Antiviral activity of low-dose alovudine in antiretroviral-experienced patients: results from a 4-week randomized, double-blind, placebo-controlled dose-ranging trial

BACKGROUND

Alovudine inhibits replication of highly nucleoside reverse transcriptase inhibitor (NRTI)-resistant HIV strains in vitro. However, dose-dependent safety concerns resulted in its initial development being halted. Recently, a 4-week course of alovudine 7.5 mg/day added to a stavudine-free failing regimen yielded a significant decrease in viral load by -1.88 log(10) HIV-1 RNA copies/mL. The magnitude of the reduction in viral load suggested that lower doses might still be effective while offering adequate safety during long-term use.

OBJECTIVE

To determine whether lower dosages of alovudine still provide significant antiviral activity in patients with broad NRTI resistance.

METHODS

A randomized, double-blind, placebo-controlled trial investigating three doses of alovudine (0.5, 1 and 2 mg) or placebo added for 4 weeks to a failing regimen in patients with evidence of NRTI-resistant HIV strains [>or=2 thymidine-associated mutations (TAMs)]. The primary endpoint was the mean viral load reduction between baseline and week 4.

RESULTS

Seventy-two patients were enrolled in the study: 21, 13, 18 and 20 in the placebo and 0.5, 1 and 2 mg arms, respectively. Baseline median CD4 count and viral load were 298 cells/microL (range 44-692 cells/microL) and 3.9 log(10) copies/mL (range 2.5-5.2 log(10) copies/mL), respectively. Baseline viral isolates harboured a median of four TAMs. Alovudine was added to a median four-drug failing regimen. At week 4, compared with placebo, mean viral load changes were -0.42 log(10) [95% confidence interval (CI) -0.67 to -0.18] and -0.30 log(10) (-0.55 to -0.06) in the 2 and 1 mg arms, respectively. There was no significant change in CD4 cell count. Alovudine was well tolerated.

CONCLUSION

A 4-week course of alovudine 2 mg/day provided a modest but significant viral load reduction in patients harbouring viruses with a median of four TAMs.

Toxicology evaluation of radiotracer doses of 3'-deoxy-3'-[18F]fluorothymidine (18F-FLT) for human PET imaging: Laboratory analysis of serial blood samples and comparison to previously investigated therapeutic FLT doses

Background

¹⁸F-FLT is a novel PET radiotracer which has demonstrated a strong potential utility for imaging cellular proliferation in human tumors in vivo. To facilitate future regulatory approval of ¹⁸F-FLT for clinical use, we wished to demonstrate the safety of radiotracer doses of ¹⁸F-FLT administered to human subjects, by: 1) performing an evaluation of the toxicity of ¹⁸F-FLT administered in radiotracer amounts for PET imaging, 2) comparing a radiotracer dose of FLT to clinical trial doses of FLT.

Methods

Twenty patients gave consent to a ¹⁸F-FLT injection, subsequent PET imaging, and blood draws. For each patient, blood samples were collected at multiple times before and after ¹⁸F-FLT PET. These samples were assayed for a comprehensive metabolic panel, total bilirubin, complete blood and platelet counts. ¹⁸F-FLT doses of 2.59 MBq/Kg with a maximal dose of 185 MBq (5 mCi) were used. Blood time-activity curves were generated for each patient from dynamic PET data, providing a measure of the area under the FLT concentration curve for 12 hours (AUC₁₂).

Results

No side effects were reported. Only albumin, red blood cell count, hematocrit and hemoglobin showed a statistically significant decrease over time. These changes are attributed to IV hydration during PET imaging and to subsequent blood loss at surgery. The AUC₁₂ values estimated from imaging data are not significantly different from those found from serial measures of FLT blood concentrations (p = 0.66). The blood samples-derived AUC₁₂ values range from 0.232 ng*h/mL to 1.339 ng*h/mL with a mean of 0.802 ± 0.303 ng*h/mL. This corresponds to 0.46% to 2.68% of the lowest and least toxic clinical trial AUC₁₂ of 50 ng*h/mL reported by Flexner et al (1994). This single injection also corresponds to a nearly 3,000-fold lower cumulative dose than in Flexner's twice daily trial.

Conclusion

This study shows no evidence of toxicity or complications attributable to ¹⁸F-FLT injected intravenously.

Randomized exposure-controlled trials; impact of randomization and analysis strategies

AIMS

In the literature, five potential benefits of randomizing clinical trials on concentration levels, rather than dose, have been proposed: (i) statistical study power will increase; (ii) study power will be less sensitive to high variability in the pharmacokinetics (PK); (iii) the power of establishing an exposure-response relationship will be robust to correlations between PK and pharmacodynamics (PD); (iv) estimates of the exposure-response relationship are likely to be less biased; and (v) studies will provide a better control of exposure in situations with toxicity issues. The main aim of this study was to investigate if these five statements are valid when the trial results are evaluated using a model-based analysis.

METHODS

Quantitative relationships between drug dose, concentration, biomarker and clinical end-point were defined using pharmacometric models. Three randomization schemes for exposure-controlled trials, dose-controlled (RDCT), concentration-controlled (RCCT) and biomarker-controlled (RBCT), were simulated and analysed according to the models.

RESULTS

(i) The RCCT and RBCT had lower statistical power than RDCT in a model-based analysis; (ii) with a model-based analysis the power for an RDCT increased with increasing PK variability; (iii) the statistical power in a model-based analysis was robust to correlations between CL and EC(50) or E(max); (iv) under all conditions the bias was negligible (<3%); and (v) for studies with equal power RCCT could produce either more or fewer adverse events compared with an RDCT.

CONCLUSION

Alternative randomization schemes may not have the proposed advantages if a model-based analysis is employed.

Emerging reverse transcriptase inhibitors for the treatment of HIV infection in adults

A combination of three or more antiretroviral drugs, commonly called 'highly active antiretroviral therapy' (HAART), has become the standard-of-care treatment for HIV-infected patients in the developed world. There are now 21 licensed anti-HIV drugs to choose from when starting a HAART regimen. The currently approved antiretroviral drugs fall into four categories: nucleoside/nucleotide reverse transcriptase inhibitors, non-nucleoside reverse transcriptase inhibitors, protease inhibitors and fusion inhibitors. Novel compounds currently in preclinical or clinical development are either focusing on new viral proteins or the same specific viral elements targeted by the available drugs. When developing new anti-HIV drugs of an already existing class, focus should be held on maximising potency, minimising toxicity, diminishing the risk for resistance development and producing effective drugs for patients who already have resistance to currently available drugs. In addition, pill burden should be ideally reduced to once-daily dosing, thereby enhancing a patient's adherence and reducing treatment costs. The present review focuses on emerging drugs to inhibit the reverse transcriptase of HIV.

Positron emission tomography in patients with breast cancer using 18F-3′-deoxy-3′-fluoro-l-thymidine (18F-FLT)—a pilot study

BACKGROUND

This pilot study investigated the feasibility of (18)F-3'-deoxy-3'-fluoro-l-thymidine ((18)F-FLT) as a positron emission tomography (PET) tracer for the visualisation of breast cancer.

METHODS

Patients with breast cancer underwent (18)F-FLT-PET prior to surgery. The uptake of (18)F-FLT was determined in the primary tumour and in the axilla.

RESULTS

Eight tumours were visualized by (18)F-FLT-PET with a mean uptake value (SUV(mean)) of 1.7 and mean tumour-non-tumour ratio (TNT) of 5.0. In seven patients, axillary lymph-node metastases were found at pathological examinations, however, (18)F-FLT-PET showed uptake in only two large (and clinically evident) lymph-node metastases.

CONCLUSIONS

(18)F-FLT shows uptake in most primary breast tumours and in large axillary lymph-node metastases.

[18F]FLT-PET in oncology: current status and opportunities

In recent years, [18F]-fluoro-3'-deoxy-3'-L: -fluorothymidine ([18F]FLT) has been developed as a proliferation tracer. Imaging and measurement of proliferation with PET could provide us with a non-invasive staging tool and a tool to monitor the response to anticancer treatment. In this review, the basis of [18F]FLT as a proliferation tracer is discussed. Furthermore, an overview of the current status of [18F]FLT-PET research is given. The results of this research show that although [18F]FLT is a tracer that visualises cellular proliferation, it also has certain limitations. In comparison with the most widely used PET tracer, [18F]FDG, [18F]FLT uptake is lower in most cases. Furthermore, [18F]FLT uptake does not always reflect the tumour cell proliferation rate, for example during or shortly after certain chemotherapy regimens. The opportunities provided by, and the limitations of, [18F]FLT as a proliferation tracer are addressed in this review, and directions are given for further research, taking into account the strong and weak points of the new tracer.

[18F]3'-deoxy-3'-fluorothymidine-PET in NHL patients: whole-body biodistribution and imaging of lymphoma manifestations--a pilot study

OBJECTIVE

The nucleoside 3'-deoxy-3'-fluorothymidine (alovudine) is an antiviral agent accumulating in proliferating cells. We prospectively evaluated the biodistribution of the PET tracer [18F]3'-deoxy-3'-fluorothymidine (FLT) and its value in detecting manifestations of non-Hodgkin's lymphoma (NHL).

METHODS

In this pilot study, 7 patients (6 male, 1 female) with indolent NHL (2), NHL in transformation (2) or aggressive NHL (3) were examined. Patients received initial staging or restaging with an interval of at least 10 weeks between therapy and positron emission tomography (PET). Mean doses of 324 +/- 165 MBq FLT were injected intravenously. Static PET scans were performed 50-70 minutes after application. Maximum standardized uptake values (SUV) of organs and NHL manifestations were calculated. FLT-positive lesions were verified by biopsies (3 lesions) or aspiration smears (5 lesions in 4 patients).

RESULTS

Biodistribution: The tracer accumulated physiologically in hematopoietic marrow and in the liver. It was renally excreted. SUV of organs 1 hour after injection were: bones with hematopoietic marrow, 9.9 +/- 4.7; liver, 5.2 +/- 1.0; kidneys, 4.0 +/- 1.7; spleen, 3.0 +/- 1.2; bones without hematopoietic marrow, 1.9 +/- 1.1; lungs, 0.8 +/- 0.3. NHL manifestations: In 7 patients, diagnosis was verified as true positive by histology/cytology. Maximum lymphoma SUV of FLT positive lesions were 6 for the indolent group and 8-11 for lymphoma in transformation. In the aggressive group, SUV were 6, 14, and 17. The low SUV in this group was found in a highly proliferate large-cell anaplastic lymphoma combined with marked sclerosis of 30%.

CONCLUSIONS

In this pilot study, FLT-PET was suitable in the imaging of NHL manifestations. In NHL with normal cellularity, FLT accumulated more intensely in aggressive NHL and NHL in transformation than in indolent NHL. Bones with hematopoietic marrow and the liver were the organs with the highest physiological uptake.

PET imaging of cellular proliferation

PET cellular proliferation imaging has its roots in a long history of in vitro cellular proliferation studies to characterize cancer and in the understanding of the biology of thymidine incorporation into DNA gained from these studies. PET imaging represents the logical translation of the in vitro work to measure in vivo tumor proliferation. Preclinical studies of [11C]-thymidine and other PET-labeled thymidine analogues set the stage for early clinical studies that provided very promising results. Recent progress in the application of [18F]-FLT, a clinically practical PET thymidine analogue, to patient studies sets the next stage for clinical PET cellular proliferation imaging. Further mechanistic studies of the imaging agents and well-designed clinical trials will be important in moving PET proliferation imaging into what is likely to be a significant role in the care of cancer patients by providing a quantitative measure of tumor response to cytotoxic or cytostatic therapy.

MIV-310 reduces HIV viral load in patients failing multiple antiretroviral therapy: results from a 4-week phase II study

BACKGROUND

Drug resistance is an increasing problem in the treatment of HIV infection. MIV-310 (alovudine), a nucleoside reverse transcriptase inhibitor, potently inhibits the replication of highly mutated strains of HIV in vitro. We examined the efficacy of MIV-310 in highly pretreated patients.

METHODS

In a phase II pilot study, 15 patients failing a current antiretroviral therapy with at least two thymidine-associated mutations (TAM) were given MIV-310 7.5 mg once daily for 4 weeks, in addition to their ongoing treatment. The primary endpoint was the plasma viral load reduction at week 4.

RESULTS

At baseline, the median viral load was 3.93 log10 copies/ml and the median CD4 cell count was 360 cells/mm3. After 4 weeks of MIV-310 administration, the median decrease in viral load was -1.13 log10. Interestingly, the median reduction was only -0.57 log10 in the four patients on stavudine, contrasting with a median reduction of -1.88 log10 in the 11 patients not receiving concomitant stavudine. The viral load fell by a median of -1.60 log10 in patients with two to three TAM (n = 7), and by -1.88 log10 in patients with four or five TAM (n = 8). The viral load rebounded in all patients after MIV-310 cessation. No mutations were found in the reverse transcriptase coding region during MIV-310 treatment. MIV-310 was well tolerated, with no serious adverse events and no treatment withdrawals.

CONCLUSION

MIV-310 7.5 mg/day efficiently reduced the HIV viral load in patients failing a multiple-drug regimen. Further studies with different dosages and longer administration times are urgently needed.

[ 18 F] 3-deoxy-3′-fluorothymidine positron emission tomography: alternative or diagnostic adjunct to 2-[ 18 f]-fluoro-2-deoxy- d -glucose positron emission tomography in the workup of suspicious central focal lesions?

BACKGROUND

2-[(18)F]-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography has been established as a standard diagnostic imaging method in the preoperative workup of suspicious pulmonary focal lesions, showing a sensitivity of more than 90% and a specificity of about 80%. Determination of malignant pulmonary lesions with FDG positron emission tomography depends on the assessment of glucose metabolism. However, false-positive findings can occur in inflammatory processes, such as sarcoidosis or pneumonia. The thymidine analogue 3-deoxy-3[(18)F]-fluorothymidine (FLT) is a new positron emission tomography tracer that more specifically targets proliferative activity of malignant lesions. The objective of this study was to determine whether FLT positron emission tomography, in comparison with FDG positron emission tomography, provides additional information in the preoperative workup of central pulmonary focal lesions.

METHODS

In this prospective study FLT and FDG positron emission tomography examinations were performed as a part of the preoperative workup in 20 patients with histologically confirmed bronchial carcinoma, 7 patients with benign lesions, and 1 patient with an atypical carcinoid. Results were compared with final pathologic findings.

RESULTS

For staging of the primary tumor, FLT positron emission tomography revealed a sensitivity of 86% and a specificity of 100% compared with a sensitivity of 95% and a specificity of 73% for FDG positron emission tomography. For N staging, the sensitivity of FLT positron emission tomography was 57% and the specificity was 100%, and for FDG positron emission tomography, the sensitivity was 86% and the specificity was 100%, respectively.

CONCLUSIONS

Our preliminary findings indicate specific FLT uptake in malignant lesions. The number of false-positive findings in FDG positron emission tomography might be reduced with FLT positron emission tomography. Therefore positron emission tomography imaging with FLT represents a useful supplement to FDG in assessing the malignancy of central pulmonary focal lesions.

Concentration-controlled or effect-controlled trials: useful alternatives to conventional dose-controlled trials?

Historically, dose-finding trials have been confirmatory in nature despite the fact that these trials represent an important and essential 'learning' phase in the drug development process. About 10 years ago 2 alternatives to the randomised dose-controlled trial (RDCT) were proposed as being more informative trial types. Controlling systemic drug exposure in order to improve efficiency of a trial forms the basis for the suggestion of a randomised concentration-controlled trial (RCCT). For the common instance where pharmacodynamic variability is larger than pharmacokinetic variability, the randomised effect-controlled trial (RECT), where patients are randomised to the effect of interest was suggested as even more informative. A survey of the literature shows that the RCCT has been sparsely applied and RECT not at all. For RCCT, the practical complications of carrying out the study seldom makes it the study type of choice. For RECT, the limited number of suitable situations for its application and the fact that the same effect is used for randomisation and analysis may explain the lack of applications. As a somewhat more favourable trial type, we suggest the randomised biomarker-controlled trial (RBCT), where patients are randomised to a certain value or range of a biomarker whereas the analysis is performed on another, clinically more relevant, effect. Although the RBCT has some attractive features, for example contributing to validation of a biomarker as a surrogate for clinical outcome, it is unlikely to be extensively used. Instead, the main shift from confirming to learning in dose-finding trials is coming from the incorporation of well-known learning components into the RDCT (e.g. sparse concentration measurements combined with population pharmacokinetic-pharmacodynamic, biomarker measurements and analysis of effect measures throughout the entire trial period).

Anti-HIV type 1 activity of 3'-fluoro-3'-deoxythymidine for several different multidrug-resistant mutants

The objective of this work was to test the antiviral activity of a potent nucleoside reverse transcriptase inhibitor, 3'-fluoro-3'-deoxythymidine (FLT), on both a wild-type human immunodeficiency virus (HIV-1) isolate and multidrug-resistant HIV-1 patient isolates. Drug-resistant viral isolates were selected on the basis of four different categories of well-characterized and representative multidrug-resistant mutants. The isolates included three variants containing 151M alone or in combination; three variants containing 215Y and 41L, 67N, 184V, 210W, and 219N in combination; two insertion mutant viruses (69 + EA and 69 + SA); and two deletion mutant viruses (del67NG and del67GS), the latter two groups both also containing other significant mutations. The activity of FLT and AZT against these isolates was determined by drug susceptibility assays and by measuring viral antigen p24 by ELISA. The cytotoxicity of FLT and AZT was assessed in PHA-stimulated PBMCs. Development of resistant mutants under FLT pressure was attempted by passaging HIV-1 isolates in SupT1 cells and stepwise increasing the concentration of FLT. The multidrug-resistant mutant HIV-1 isolates exhibited 7-fold to >100-fold increased resistance to AZT, but showed IC(50) values for FLT of 0.0014-0.0168 microM, which were lower than or similar to that of wild type (0.0075 microM). The cellular cytotoxicities of FLT and AZT fell into a similar range in PBMCs. The development of HIV mutants resistant to FLT appeared to be slower than for other RT inhibitors. HIV isolates with mutations resulting in multidrug resistance had no evidence of resistance to FLT. FLT may be useful in salvage therapies for patients harboring resistant strains and a reassessment of its therapeutic potential seems required.

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.

Simulation of clinical trials

Computer simulation of clinical trials has evolved over the past two decades from a simple instructive game to "full" simulation models yielding pharmacologically sound, realistic trial outcomes. The need to make drug development more efficient and informative and the awareness that many industries make extensive use of simulation in product development have advanced considerably the use of simulation of clinical trials in pharmaceutical product development over the past decade. The structural and stochastic components of trial simulation models are explained as a prelude to a listing of representative simulation projects, reflecting investigative applications of statistical methods, trial design comparisons, and full simulation of new drugs being developed. Lessons learned from these projects are reviewed in the context of their current impact and potential for influencing the future of drug development.

Synthesis of 2',3'-dideoxy-3'-fluoro-L-ribonucleosides as potential antiviral agents from D-sorbitol

2',3'-Dideoxy-3'-fluoro-L-ribonucleosides were synthesized as potential antiviral agents. The key intermediate, methyl 5-O-benzoyl-2,3-dideoxy-3-fluoro-L-ribofuranoside, which was prepared from D-sorbitol, was condensed with pyrimidine and purine bases to obtain the respective nucleosides. Among them, the cytosine analogue 2',3'-dideoxy-3'-fluoro-alpha-L-cytidine showed a moderate anti-HBV activity.

Population pharmacokinetics and pharmacodynamic modeling of abacavir (1592U89) from a dose-ranging, double-blind, randomized monotherapy trial with human immunodeficiency virus-infected subjects

Abacavir (formerly 1592U89) is a carbocyclic nucleoside analog with potent anti-human immunodeficiency virus (anti-HIV) activity when administered alone or in combination with other antiretroviral agents. The population pharmacokinetics and pharmacodynamics of abacavir were investigated in 41 HIV type 1 (HIV-1)-infected, antiretroviral naive adults with baseline CD4(+) cell counts of >/=100/mm(3) and plasma HIV-1 RNA levels of >30,000 copies/ml. Data for analysis were obtained from patients who received randomized, blinded monotherapy with abacavir at 100, 300, or 600 mg twice-daily (BID) for up to 12 weeks. Plasma abacavir concentrations from sparse sampling were analyzed by standard population pharmacokinetic methods, and the effects of dose, combination therapy, gender, weight, and age on parameter estimates were investigated. Bayesian pharmacokinetic parameter estimates were calculated to determine the peak concentration of abacavir in plasma (C(max)) and the area under the concentration-time curve from time zero to infinity (AUC(0-infinity)) for individual subjects. The pharmacokinetics of abacavir were dose proportional over the 100- to 600-mg dose range and were unaffected by any covariates. No significant correlations were observed between the incidence of the five most common adverse events (headache, nausea, diarrhea, vomiting, and malaise or fatigue) and AUC(0-infinity). A significant correlation was observed between C(max) and nausea by categorical analysis (P = 0.019), but this was of borderline significance by logistic regression (odds ratio, 1.45; 95% confidence interval, 0.95 to 2.32). The log(10) time-averaged AUC(0-infinity) minus baseline (AAUCMB) values for HIV-1 RNA and CD4(+) cell count correlated significantly with C(max) and AUC(0-infinity), but with better model fits for AUC(0-infinity). The increase in AAUCMB values for CD4(+) cell count plateaued early for drug exposures that were associated with little change in AAUCMB values for plasma HIV-1 RNA. There was less than a 0.4 log(10) difference over 12 weeks in the HIV-1 RNA levels with the doubling of the abacavir AUC(0-infinity) from 300 to 600 mg BID dosing. In conclusion, pharmacodynamic modeling supports the selection of abacavir 300 mg twice-daily dosing.

Characterizing tumors using metabolic imaging: PET imaging of cellular proliferation and steroid receptors

Treatment decisions in oncology are increasingly guided by information on the biologic characteristics of tumors. Currently, patient-specific information on tumor biology is obtained from the analysis of biopsy material. Positron emission tomography (PET) provides quantitative estimates of regional biochemistry and receptor status and can overcome the sampling error and difficulty in performing serial studies inherent with biopsy. Imaging using the glucose metabolism tracer, 2 -deoxy-2- fluoro-D-glucose (FDG), has demonstrated PET's ability to guide therapy in clinical oncology. In this review, we highlight PET approaches to imaging two other aspects of tumor biology: cellular proliferation and tumor steroid receptors. We review the biochemical and biologic processes underlying the imaging, positron-emitting radiopharmaceuticals that have been developed, quantitative image-analysis considerations, and clinical studies to date. This provides a basis for evaluating future developments in these promising applications of PET metabolic imaging.

Imaging proliferation in vivo with [F-18]FLT and positron emission tomography

Positron emission tomography (PET) is now regularly used in the diagnosis and staging of cancer. These uses and its ability to monitor treatment response would be aided by the development of imaging agents that can be used to measure tissue and tumor proliferation. We have developed and tested [F-18]FLT (3'-deoxy-3'-fluorothymidine); it is resistant to degradation, is retained in proliferating tissues by the action of thymidine kinase 1 (TK), and produces high-contrast images of normal marrow and tumors in canine and human subjects.

Newly synthesized L-enantiomers of 3'-fluoro-modified beta-2'-deoxyribonucleoside 5'-triphosphates inhibit hepatitis B DNA polymerases but not the five cellular DNA polymerases alpha, beta, gamma, delta, and epsilon nor HIV-1 reverse transcriptase

Novel beta-L-2',3'-dideoxy-3'-fluoro nucleosides were synthesized and further converted to their 5'-triphosphates. Their inhibitory activities against hepatitis B virus (HBV) and duck hepatitis B virus (DHBV) DNA polymerases, human immunodeficiency virus (HIV) reverse transcriptase (RT), and the cellular DNA polymerases alpha, beta, gamma, delta, and epsilon were investigated and compared with those of the corresponding 3'-fluoro-modified beta-d-analogues. The 5'-triphosphates of 3'-deoxy-3'-fluoro-beta-L-thymidine (beta-L-FTTP), 2',3'-dideoxy-3'-fluoro-beta-L-cytidine (beta-L-FdCTP), and 2',3'-dideoxy-3'-fluoro-beta-l-5-methylcytidine (beta-L-FMetdCTP) emerged as effective inhibitors of HBV/DHBV DNA polymerases (IC50 = 0.25-10.4 microM). They were either equally (FTTP) or less (FMetdCTP, FdCTP) effective than their beta-d-counterparts. Also the 5'-triphosphate of beta-L-thymidine (beta-L-TTP) was shown to be a strong inhibitor of these two viral enzymes (IC50 = 0.46/1.0 microM). However, all beta-L-FdNTPs (also beta-L-TTP) were inactive against HIV-RT, a result which contrasts sharply with the high efficiency of the beta-D- FdNTPs against this polymerase. Between the cellular DNA polymerases only the beta and gamma enzymes displayed a critical susceptibility to beta-D-FdNTPs which is largely abolished by the beta-L-enantiomers. These results recommend beta-L-FTdR, beta-L-FCdR, and beta-L-FMetCdR for further evaluation as selective inhibitors of HBV replication at the cellular level.

Drug compliance in therapeutic trials: a review

Because poor compliance introduces a major risk of bias in the interpretation of the results of a therapeutic trial, it is an important element to consider. At the planning stage, factors known to be associated with poor compliance should be recognized. The different methods of evaluating compliance, either clinical or biological, should be reviewed and the best strategy selected. During the therapeutic trial, the objective is to maintain an appropriate level of compliance. Patients, investigators, and sponsors have different options and responsibilities. The analysis should incorporate compliance as a specific variable in order to help test the robustness of the data. Compliance constitutes by itself a specific outcome measure. Compliance should be an integral part of study reports and publications, but it is frequently not discussed.

Principles of clinical pharmacology in postexposure prophylaxis

Deciding on postexposure prophylaxis for any infection requires that the patient and healthcare provider understand the magnitude of infection risk and the adverse consequences of therapeutic intervention or nonintervention. Principles of epidemiology and microbiology allow us to estimate the risk of infection. Principles of clinical pharmacology allow us to estimate the risk and benefit of therapy. The dose-response-time relationships for antiviral activity and toxicity of a drug can be used to develop regimens that maximize benefit and minimize risk. Other important pharmacologic considerations include the role of active and toxic drug metabolites, combination chemotherapy, drug interactions, and medication compliance.

Conformational analysis of the complete series of 2' and 3' monofluorinated dideoxyuridines

The solution conformations of a set of uridine 2',3'-dideoxynucleosides, where each of the hydrogens at the 2'- and 3'-positions of the sugar ring were individually replaced with a fluorine atom, were studied by nuclear magnetic resonance spectroscopy and pseudorotational analysis. The distribution of the north/south (N/S) puckering equilibrium for each compound was calculated by coupling constant analysis aided by the program PSEUROT. The data confirmed that the pseudorotational equilibrium of the fluorinated glycones is governed by the position of the fluorine atom. The preferred rotamer populations about the C4'-C5' (gamma) and C1'-N1' (chi) bonds calculated from coupling constant and NOE analysis, respectively, were also influenced by the presence of fluorine. Proton coupling to the fluorine atom was also used to qualitatively estimate the N/S equilibrium population. Through space, long range 1H-19F coupling constants were observed in compounds where the fluorine atom was above the plane of the ring ('up'). The pseudorotational parameters of the compounds described were tempered by the anomeric effect which drives the pseudorotational equilibrium towards the 2'-exo/3'-endo (northern) pucker. Ab initio calculations using the 3-21 G* basis set yielded a measure of the energy differences between the N and S local minima in each compound. These results agree with previous conformational studies of other fluorinated nucleoside analogues and prove that the furanose ring pucker is governed by the highly electronegative fluorine atom. However, the competing anomeric effect plays a major role in determining the mole fraction of the minor conformer of these compounds in solution.

Selective endocytosis of fluorothymidine and azidothymidine coupled to LDL into HIV infected mononuclear cells

Drug targeting via lipoproteins may be of benefit for use of cytotoxic drugs like fluorothymidine (FLT) or azidothymidine (AZT). Both drugs are potent inhibitors of the human immunodeficiency virus (HIV) reverse transcriptase and are used in the therapy of HIV infection. With regard to this project, the selective endocytosis in HIV infected human macrophages was studied after covalent coupling of AZT and LDL to low density lipoproteins (LDL). Cultured human macrophages and the lymphocytic Molt 4/8 cell line were infected with HIV-1 in vitro and subsequently treated with FLT-LDL or AZT-LDL. Viral replication was followed by determination of cell-released capsid antigen p24. Internalisation into HIV-1 infected human macrophages by the scavenger receptor pathway leads to a dose dependent inhibition of HIV replication. Otherwise, in HIV infected, but scavenger receptor missing lymphocytes (Molt 4/8 cells), neither endocytosis nor inhibition of HIV replication results. Thus, covalent coupling of drugs to LDL leads to a macrophage specific transport. This strategy could possibly avoid toxic side effects in the therapeutic use of antiretroviral drugs and thus may open a way for an earlier chemotherapy in HIV infection.

Antiretroviral drugs for AIDS

Treatment of HIV disease with antiretroviral agents has changed considerably. We now know that monotherapy is not the best strategy in most cases to combat rapid turnover of virus and development of resistance (the exception being mother-to-child transmission) and various combination drug regimens are being explored. Apart from the main drug groups, consisting of nucleoside analogues, proteinase inhibitors, and reverse-transcriptase inhibitors, many new compounds are under development. The timing of therapy may likewise be important, and the indications of benefit from early initiation of treatment need to be confirmed in randomised trials. Overall, there is far more optimism about the use of drugs in HIV infection than there was several years ago.

Pharmacologic considerations for antiviral drug development

OBJECTIVE

To discuss pharmacologic considerations for the development of antiviral agents.

DATA SOURCES

English-language literature pertaining to the development and clinical evaluation of antiviral compounds, primarily agents targeted against herpes group viruses and HIV.

STUDY SELECTION AND DATA EXTRACTION

Pertinent information, as judged by the author, was selected for discussion.

DATA SYNTHESIS

Drug development of antiviral agents presents unique problems compared with that of antimicrobial and other agents. Understanding the mechanism of action and both pharmacokinetic and pharmacodynamic considerations is critical to developing a rational dosing strategy and safe, effective use. The lack of standardized methods for antiviral susceptibility testing and the influence of factors such as strain of virus, host cell type, culture medium, inoculum size, end point, and method of measurement on the results obtained illustrate factors that complicate preclinical pharmacologic analysis of antiviral agents. Acyclovir offers a model for clinical drug development. Its mechanism of action, pharmacokinetics, and pharmacodynamics have been studied extensively. Rational guidelines for usage are available, including guidelines in special patient populations such as kidney transplant recipients and neonates. A pregnancy registry has allowed evaluation of the incidence of birth defects in fetuses exposed to systemic acyclovir. Several pitfalls in antiviral drug development are associated with inadequate pharmacologic information. The development of dextran sulfate and fialuridine provides two examples. Integration of pharmacokinetic and pharmacodynamic analyses using modern sampling and analysis techniques may facilitate more rapid development of antiviral agents and more informed dosage regimens to achieve the highest probability of therapeutic success.