Lack of a pharmacokinetic interaction between a new smoking cessation therapy, varenicline, and digoxin in adult smokers

Ultrapotent chemogenetics for research and potential clinical applications

Chemogenetics enables noninvasive chemical control over cell populations in behaving animals. However, existing small-molecule agonists show insufficient potency or selectivity. There is also a need for chemogenetic systems compatible with both research and human therapeutic applications. We developed a new ion channel-based platform for cell activation and silencing that is controlled by low doses of the smoking cessation drug varenicline. We then synthesized subnanomolar-potency agonists, called uPSEMs, with high selectivity for the chemogenetic receptors. uPSEMs and their receptors were characterized in brains of mice and a rhesus monkey by in vivo electrophysiology, calcium imaging, positron emission tomography, behavioral efficacy testing, and receptor counterscreening. This platform of receptors and selective ultrapotent agonists enables potential research and clinical applications of chemogenetics.

Physiological based pharmacokinetic modeling to estimate in vivo Ki of ketoconazole on renal P-gp using human drug-drug interaction study result of fesoterodine and ketoconazole

This study was conducted to estimate in vivo inhibition constant (Ki) of ketoconazole on renal P-glycoprotein (P-gp) using human drug-drug interaction (DDI) study result of fesoterodine and ketoconazole. Fesoterodine is a prodrug which is extensively hydrolyzed by non-specific esterases to the active metabolite 5-hydroxymethyl tolterodine (5-HMT). 5-HMT is then further metabolized via Cytochrome P450 (CYP) 2D6 and CYP3A4. It is reported that 5-HMT is a substrate of P-gp whereas fesoterodine is not. Renal clearance of 5-HMT is approximately two-times greater than renal glomerular filtration rate. This suggests the possibility that renal clearance of 5-HMT involves secretion by P-gp. Utilizing the available pharmacokinetic characteristics of fesoterodine and 5-HMT, we estimated in vivo Ki of ketoconazole on P-gp at kidney based on DDI study data using physiologically-based pharmacokinetic approach. The estimated in vivo Ki of ketoconazole for hepatic CYP3A4 (6.64 ng/mL) was consistent with the reported values. The in vivo Ki of ketoconazole for renal P-gp was successfully estimated as 2.27 ng/mL, which was notably lower than reported in vitro 50% inhibitory concentration (IC₅₀) values ranged 223-2440 ng/mL due to different condition between in vitro and in vivo.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

In vitro predictability of drug-drug interaction likelihood of P-glycoprotein-mediated efflux of dabigatran etexilate based on [I]2/IC50 threshold

Dabigatran etexilate, an oral, reversible, competitive, and direct thrombin inhibitor, is an in vitro and in vivo substrate of P-glycoprotein (P-gp). Dabigatran etexilate was proposed as an in vivo probe substrate for intestinal P-gp inhibition in a recent guidance on drug-drug interactions (DDI) from the European Medicines Agency (EMA) and the Food and Drug Administration (FDA). We conducted transcellular transport studies across Caco-2 cell monolayers with dabigatran etexilate in the presence of various P-gp inhibitors to examine how well in vitro IC50 data, in combination with mathematical equations provided by regulatory guidances, predict DDI likelihood. From a set of potential P-gp inhibitors, clarithromycin, cyclosporin A, itraconazole, ketoconazole, quinidine, and ritonavir inhibited P-gp-mediated transport of dabigatran etexilate over a concentration range that may hypothetically occur in the intestine. IC50 values of P-gp inhibitors for dabigatran etexilate transport were comparable to those of digoxin, a well established in vitro and in vivo P-gp substrate. However, IC50 values varied depending whether they were calculated from efflux ratios or permeability coefficients. Prediction of DDI likelihood of P-gp inhibitors using IC50 values, the hypothetical concentration of P-gp inhibitors, and the cut-off value recommended by both the FDA and EMA were in line with the DDI occurrence in clinical studies with dabigatran etexilate. However, it has to be kept in mind that validity of the cut-off criteria proposed by the FDA and EMA depends on in vitro experimental systems and the IC50-calculation methods that are employed, as IC50 values are substantially influenced by these factors.

Disposition and drug-drug interaction potential of veliparib (ABT-888), a novel and potent inhibitor of poly(ADP-ribose) polymerase

The disposition of veliparib [(R)-2-(2-methylpyrrolidin-2-yl)-1H-benzo[d]imidazole-4-carboxamide, ABT-888], a novel and potent inhibitor of poly(ADP-ribose) polymerase for the treatment of cancers, was investigated in rats and dogs after intravenous and oral administration of [(3)H]veliparib and compared with that of humans. Veliparib absorption was high. Dosed radioactivity was widely distributed in rat tissues. The majority of drug-related material was excreted in urine as unchanged drug (approximately 54, 41, and 70% of the dose in rats, dogs, and humans, respectively). A lactam M8 and an amino acid M3 were two major excretory metabolites in animals. In the circulation of animals and humans, veliparib was the major drug-related component, and M8 was one of the major metabolites. Monooxygenated metabolite M2 was significant in the rat and dog, and M3 was also significant in the dog. Veliparib biotransformation occurred on the pyrrolidine moiety via formation of a lactam, an amino acid, and an N-carbamoyl glucuronide, in addition to oxidation on benzoimidazole carboxamide and sequential glucuronidation. In vitro experiments using recombinant human cytochrome P450 (P450) enzymes identified CYP2D6 as the major enzyme metabolizing veliparib with minor contributions from CYP1A2, 2C19, and 3A4. Veliparib did not inhibit or induce the activities of major human P450s. Veliparib was a weak P-glycoprotein (P-gp) substrate, showing no P-gp inhibition. Taken together, these studies indicate a low potential for veliparib to cause clinically significant P-gp or P450-mediated drug-drug interactions (DDIs). Overall, the favorable dispositional and DDI profiles of veliparib should be beneficial to its safety and efficacy.

Varenicline: a novel pharmacotherapy for smoking cessation

Varenicline is an orally administered small molecule with partial agonist activity at the alpha4beta2 nicotinic acetylcholine receptor. Varenicline was approved by both the US FDA and the European Medicines Agency of the EU in 2006 as an aid to smoking cessation. Subsequently, varenicline has been approved in over 80 other countries. Varenicline is almost entirely absorbed following oral administration, and absorption is unaffected by food, smoking or the time of day. Varenicline undergoes only minimal metabolism and approximately 90% of the drug is excreted in the urine unchanged. Varenicline has a mean elimination half-life after repeated administration of approximately 24 hours in smokers. The area under the plasma concentration-time curve is increased in patients with moderate or severe renal failure. No clinically relevant varenicline-drug interactions have been identified. In two identical, randomized, double-blind, phase III clinical trials in healthy, motivated-to-quit, mainly Caucasian smokers aged 18-75 years in the US, 12 weeks of treatment with varenicline 1 mg twice daily was associated with significantly higher abstinence rates over weeks 9-12 than sustained-release bupropion 150 mg twice daily or placebo. In a separate phase III trial, an additional 12 weeks of treatment in smokers achieving abstinence in the first 12 weeks was associated with greater abstinence through to week 52 than placebo treatment. Varenicline treatment was also associated with significantly higher rates of abstinence than placebo treatment in randomized, double-blind, clinical trials in smokers in China, Japan, Korea, Singapore, Taiwan and Thailand. In a randomized, open-label, multi-national, phase III trial, varenicline treatment was associated with a significantly higher rate of abstinence than transdermal nicotine-replacement therapy. In these trials, varenicline treatment was associated with lower urge to smoke and satisfaction from smoking in relapsers than placebo or active comparators. In the two US phase III trials, 12 weeks of treatment with varenicline 1 mg twice daily had an acceptable safety and tolerability profile. Nausea and abnormal dreams were the most common adverse events that occurred in more varenicline than placebo recipients. The incidence and prevalence of nausea were greatest in weeks 1 and 2 of treatment, and declined thereafter. The prevalence of early adverse effects can be reduced by individual dose titration. Adverse events associated with varenicline therapy have been reported in post-marketing surveillance, including neuropsychiatric events such as depressed mood, agitation, changes in behaviour, suicidal ideation and suicide. Currently, it is unclear whether the association of varenicline therapy with these adverse events is causal, coincidental or related to smoking cessation. Given the greater efficacy of varenicline compared with other pharmacotherapies, and the high risk of morbidity and mortality associated with continued smoking, varenicline is a valuable pharmacological aid to smoking cessation.