Rajesh Krishna

Multidrug resistance (MDR) in cancer. Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs.

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition, its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated in various studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR), reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokinetic interactions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first and second generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible in eliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDR modulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution, clearance, and interactions) will also be discussed.

Effect of the cholesteryl ester transfer protein inhibitor, anacetrapib, on lipoproteins in patients with dyslipidaemia and on 24-h ambulatory blood pressure in healthy individuals: two double-blind, randomised placebo-controlled phase I studies

BACKGROUND The inhibition of cholesteryl ester transfer protein (CETP) is considered a potential new mechanism for treatment of dyslipidaemia. Anacetrapib (MK-0859) is a CETP inhibitor currently under development. We aimed to assess anacetrapibs effects as monotherapy on low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C) and on 24-h ambulatory blood pressure. METHODS We did two double-blind, randomised, placebo-controlled phase I studies. In the first study, 50 patients with dyslipidaemia (LDL-C 100-190 mg/dL; 40 active, 10 placebo) aged 18-75 years received anacetrapib doses of 0, 10, 40, 150, or 300 mg orally once a day with a meal for 28 days. Standard lipid and lipoprotein monitoring, safety monitoring, and anacetrapib concentrations for pharmacokinetics were done. In the second study, 22 healthy participants aged 45-75 years received either 150 mg of anacetrapib once a day or matching placebo with a meal for 10 days in each crossover period, in a randomised sequence, with at least a 14-day washout between the treatment periods. Continuous 24-h ambulatory blood pressure monitoring was done on day -1 and day 10 of each treatment period in this study. The primary or secondary endpoints of safety and tolerability were assessed in both studies by monitoring clinical adverse experiences, physical examinations, vital signs, 12-lead electrocardiogram, and laboratory safety. Analysis was per protocol. These trials are registered with ClinicalTrials.gov, number NCT00565292 and NCT00565006. FINDINGS In the dyslipidaemia study, one patient withdrew consent and one was excluded from the data analysis for HDL-C and LDL-C because complete pre-dose measurements were not available. Anacetrapib produced dose-dependent lipid-altering effects with peak lipid-altering effects of 129% (mean 51.1 [SD 3.8]-114.9 [7.9] mg/dL) increase in HDL-C and a 38% (138.2 [11.4]-77.6 [7.9] mg/dL) decrease in LDL-C in patients with dyslipidaemia. In the 24-h ambulatory blood pressure study in healthy individuals, least squares difference between anacetrapib and placebo groups on day 10 were 0.60 (90% CI -1.54 to 2.74; p=0.634) mm Hg for systolic blood pressure and 0.47 (90% CI -0.90 to 1.84; p=0.561) mm Hg for diastolic blood pressure. INTERPRETATION Anacetrapib seems to exhibit HDL-C increases greater than those seen with other investigational drugs in this class and LDL-C lowering effects similar to statins. Despite greater lipid-altering effects relative to other members of this class, anacetrapib seems not to increase blood pressure, suggesting that potent CETP inhibition by itself might not lead to increased blood pressure.

Multiple-dose pharmacodynamics and pharmacokinetics of anacetrapib, a potent cholesteryl ester transfer protein (CETP) inhibitor, in healthy subjects.

Cholesteryl ester transfer protein (CETP) is a plasma protein that catalyzes the heteroexchange of cholesteryl esters from high‐density lipoprotein (HDL) and triglycerides to apolipoprotein B–containing lipoproteins, especially very low–density lipoproteins (LDL‐C). 1, 2

Anacetrapib, a Novel CETP Inhibitor: Pursuing a New Approach to Cardiovascular Risk Reduction

Cholesteryl ester transfer protein (CETP) inhibition is a promising experimental strategy to raise high‐density lipoprotein cholesterol (HDL‐C) and reduce cardiovascular risk. This review focuses on the highly selective and potent CE TP inhibitor anacetrapib and discusses the available preclinical and clinical information pertaining to it. We also describe strategies to target HDL‐C, discuss the mechanism underlying CETP inhibition and its effects on lipid biology, and give an overview of other CETP inhibitors that are currently in development.

Potent and Selective Agonism of the Melanocortin Receptor 4 With MK-0493 Does Not Induce Weight Loss in Obese Human Subjects: Energy Intake Predicts Lack of Weight Loss Efficacy

MK‐0493 is a novel, potent, and selective agonist of the melanocortin receptor 4 (MC4R), one of the best‐validated genetic targets and considered one of the most promising for the development of antiobesity therapeutics. An ad libitum energy‐intake model was qualified with excellent reproducibility: the geometric mean ratio (GMR) with 95% confidence interval (CI) for total energy intake over a period of 24 h for 30 mg sibutramine/placebo was 0.82 (0.76, 0.88), and for 10 mg sibutramine/placebo it was 0.98 (0.91, 1.05). MK‐0493 showed a small and marginally significant effect on 24‐h energy intake, whereas 30 mg of sibutramine caused a significant reduction in total 24‐h energy intake; specifically, the GMR (95% CI) for 30 mg sibutramine/placebo was 0.79 (0.74, 0.85). MK‐0493 was associated with modest weight reduction from baseline but had only small, statistically insignificant effects relative to placebo after 12 weeks in a fixed‐dose study and also after 18 weeks of stepped‐titration dosing. We conclude that agonism of MC4R is not likely to represent a viable approach to the development of antiobesity therapeutics.

Modulation of P-Glycoprotein (PGP) Mediated Multidrug Resistance (MDR) Using Chemosensitizers: Recent Advances in the Design of Selective MDR Modulators

Over the past two decades, a number of chemical entities have been investigated in the continuing quest to reverse P-glycoprotein (PGP) mediated multidrug resistance (MDR) in cancer. The complexity of interactions between these agents and the proteins responsible for MDR in conjunction with the challenges associated with developing SAR/QSAR relationships for MDR modulators has hampered our ability to develop agents that modulate MDR with enhanced specificity of target, increased efficacy, and minimized toxicity when coadministered with anticancer drugs. With an increased understanding of the molecular interaction, target-mediated SAR and combinatorial chemistry approaches, newer more selective inhibitors have been recently reported. These agents have shown remarkable promise in preclinical trials; although their ultimate clinical therapeutic utility remains to be established. The emphasis of this review is placed on the current understanding of modulator-drug transport protein interactions and to review the advances in the structure-based design, synthetic efforts and the cellular pharmacology of MDR modulating activity of a number of known PGP inhibitors.

Single‐dose pharmacokinetics and pharmacodynamics of anacetrapib, a potent cholesteryl ester transfer protein (CETP) inhibitor, in healthy subjects

AIMS Anacetrapib is an orally active and potent inhibitor of CETP in development for the treatment of dyslipidaemia. These studies endeavoured to establish the safety, tolerability, pharmacokinetics and pharmacodynamics of rising single doses of anacetrapib, administered in fasted or fed conditions, and to preliminarily assess the effect of food, age, gender and obesity on the single-dose pharmacokinetics and pharmacodynamics of anacetrapib. METHODS Safety, tolerability, anacetrapib concentrations and CETP activity were evaluated. RESULTS Anacetrapib was rapidly absorbed, with peak concentrations occurring at approximately 4 h post-dose and an apparent terminal half-life ranging from approximately 9 to 62 h in the fasted state and from approximately 42 to approximately 83 h in the fed state. Plasma AUC and C(max) appeared to increase in a less than approximately dose-dependent manner in the fasted state, with an apparent plateau in absorption at higher doses. Single doses of anacetrapib markedly and dose-dependently inhibited serum CETP activity with peak effects of approximately 90% inhibition at t(max) and approximately 58% inhibition at 24 h post-dose. An E(max) model best described the plasma anacetrapib concentration vs CETP activity relationship with an EC(50) of approximately 22 nm. Food increased exposure to anacetrapib; up to approximately two-three-fold with a low-fat meal and by up to approximately six-eight fold with a high-fat meal. Anacetrapib pharmacokinetics and pharmacodynamics were similar in elderly vs young adults, women vs men, and obese vs non-obese young adults. Anacetrapib was well tolerated and was not associated with any meaningful increase in blood pressure. CONCLUSIONS Whereas food increased exposure to anacetrapib significantly, age, gender and obese status did not meaningfully influence anacetrapib pharmacokinetics and pharmacodynamics.

Effect of a Single Cyclosporine Dose on the Single‐Dose Pharmacokinetics of Sitagliptin (MK‐0431), a Dipeptidyl Peptidase‐4 Inhibitor, in Healthy Male Subjects

Sitagliptin (MK‐0431) is an orally active, potent, and selective dipeptidyl peptidase‐4 inhibitor used for the treatment of patients with type 2 diabetes mellitus. Sitagliptin has been shown to be a substrate for P‐glycoprotein in preclinical studies. Cyclosporine was used as a probe P‐glycoprotein inhibitor at a high dose to evaluate the potential effect of potent P‐glycoprotein inhibition on single‐dose sitagliptin pharmacokinetics in healthy male subjects. Eight healthy young men received a single oral 600‐mg dose of cyclosporine with a single 100‐mg oral sitagliptin dose and a single oral 100‐mg sitagliptin dose alone in an open‐label, randomized, 2‐period, crossover study. Single doses of sitagliptin with or without single doses of cyclosporine were generally well tolerated. The sitagliptin AUC0‐∞ geometric mean ratio was 1.29 with a 90% confidence interval of (1.24, 1.34). The sitagliptin Cmax geometric mean ratio was 1.68 with a 90% confidence interval of (1.35, 2.08). Cyclosporine coadministration did not appear to affect apparent sitagliptin renal clearance, t1/2, or C24 h, suggesting that effects of these high doses of cyclosporine are more likely due to enhanced absorption of sitagliptin, potentially through inhibition of intestinal P‐glycoprotein. These results rationalize the use of a single high‐dose cyclosporine as a probe inhibitor of P‐glycoprotein for compound candidates whose elimination is less dependent on CYP3A4‐mediated metabolism.

Accelerating Drug Development Using Biomarkers: A Case Study with Sitagliptin, A Novel DPP4 Inhibitor for Type 2 Diabetes

The leveraged use of biomarkers presents an opportunity in understanding target engagement and disease impact while accelerating drug development. For effective integration in drug development, it is essential for biomarkers to aid in the elucidation of mechanisms of action and disease progression. The recent years have witnessed significant progress in biomarker selection, validation, and qualification, while enabling surrogate and clinical endpoint qualification and application. Biomarkers play a central role in target validation for novel mechanisms. They also play a central role in the learning/confirming paradigm, particularly when utilized in concert with pharmacokinetic/pharmacodynamic modeling. Clearly, these attributes make biomarker integration attractive for scientific and regulatory applications to new drug development. In this review, applications of proximal, or target engagement, and distal, or disease-related, biomarkers are highlighted using the example of the recent development of sitagliptin for type 2 diabetes, wherein elucidation of target engagement and disease-related biomarkers significantly accelerated sitagliptin drug development. Importantly, use of biomarkers as tools facilitated design of clinical efficacy trials while streamlining dose focus and optimization, the net impact of which reduced overall cycle time to filing as compared to the industry average.

Assessment of the CYP3A‐Mediated Drug Interaction Potential of Anacetrapib, a Potent Cholesteryl Ester Transfer Protein (CETP) Inhibitor, in Healthy Volunteers

In this study, midazolam was used as a probe‐sensitive CYP3A substrate to investigate the effect of anacetrapib on CYP3A activity, and ketoconazole was used as a probe‐inhibitor to investigate the effect of potent CYP3A inhibition on the pharmacokinetics of anacetrapib, a novel cholesteryl ester transfer protein inhibitor in development for the treatment of dyslipidemia. Two partially blinded, randomized, 2‐period, fixed‐sequence studies were performed. Safety, tolerability, and midazolam and anacetrapib plasma concentrations were assessed. All treatments were generally well tolerated. The geometric mean ratios (90% confidence interval) of midazolam with anacetrapib/midazolam alone for AUC0‐∞ and Cmax were 1.04 (0.94, 1.14) and 1.15 (0.97, 1.37), respectively. Exposure to anacetrapib was increased by ketoconazole—specifically, the geometric mean ratios (90% confidence interval) of anacetrapib with ketoconazole/anacetrapib alone for AUC0‐∞ and Cmax were 4.58 (3.68, 5.71) and 2.37 (2.02, 2.78), respectively. The study showed that anacetrapib does not inhibit or induce CYP3A activity. Furthermore, anacetrapib appears to be a moderately sensitive substrate of CYP3A.