Safety Pharmacology Screening: Practical Problems in Drug Development

Methylation Products of 6β- N-Heterocyclic Substituted Naltrexamine Derivatives as Potential Peripheral Opioid Receptor Modulators

Two 6β- N-heterocyclic naltrexamine derivatives, NAP and NMP, have been identified as peripherally selective mu opioid receptor (MOR) antagonists. To further enhance the peripheral selectivity of both compounds, the 17-amino group and the nitrogen atom of the pyridine ring in both NAP and NMP were methylated to obtain dMNAP and dMNMP, respectively. Compared with NAP and NMP, the binding affinities of dMNAP and dMNMP shifted to MOR and KOR (kappa opioid receptor) dual selective and they acted as moderate efficacy partial agonists. The results from radioligand binding studies were further confirmed by molecular docking studies. In vivo studies demonstrated that dMNAP and dMNMP did not produce antinociception nor did they antagonize morphine's antinociceptive activity, indicating that these compounds did not act on the central nervous system. Meanwhile, both dMNAP and dMNMP significantly slowed down fecal excretion, which indicated that they were peripherally acting opioid receptor agonists. All together, these results suggested that dMNAP and dMNMP acted as peripheral mu/kappa opioid receptor modulators and may be applicable in the treatment of diarrhea in patients with bowel dysfunction.

6β-N-heterocyclic substituted naltrexamine derivative NAP as a potential lead to develop peripheral mu opioid receptor selective antagonists

A 6β-N-heterocyclic substituted naltrexamine derivative, NAP, was proposed as a peripheral mu opioid receptor (MOR) selective antagonist based on the in vitro and in vivo pharmacological and pharmacokinetic studies. To further validate this notion, several functional assays were carried out to fully characterize this compound. In the charcoal gavage and intestinal motility assay in morphine-pelleted mice, when administered 0.3 mg/kg or higher doses up to 3 mg/kg subcutaneously, NAP significantly increased the intestinal motility compared to the saline treatment. The comparative opioid withdrawal precipitation study and the lower locomotor assay demonstrated that NAP showed only marginal intrinsic effect in the central nervous system either given subcutaneously or intravenously: no jumps were witnessed for the tested animals even given up to a dose of 50 mg/kg, while similar noticeable wet-dog shakes only occurred at the dose 50 times of those for naloxone or naltrexone, and significant reduction of the hyper-locomotion only happened at the dose as high as 32 mg/kg. Collectively, these results suggested that NAP may serve as a novel lead to develop peripheral MOR selective antagonist which might possess therapeutic potential for opioid-induced bowel dysfunction (OBD), such as opioid-induced constipation (OIC).

Effects of fasting on evaluation of gastrointestinal transit with charcoal meal

INTRODUCTION

At the present time, most studies investigating gastrointestinal transit time with charcoal are conducted in fasted rats. It seems reasonable to hypothesize that the fasting state of rats could influence the effect a compound had on gastrointestinal transit time. The purpose of this study was to investigate the effects of food on the pharmacological effects on gastrointestinal transit.

METHODS

For each drug investigated, two sets of 32 male Sprague-Dawley rats were used. One set was studied after being fasted for approximately 6 h, the second set was studied after free access to food. Each set had 4 groups of animals (n=8/group) that were administered different doses, allowing the assessment of the drug effect after fasting and after free access to food. Animals were administered 0, 10, 25, and 75 mg/kg of morphine; 0, 10, 20, and 40 mg/kg loperamide, or 0, 0.05, 0.5, and 3.0 mg/kg clonidine. At predetermined times, an activated charcoal suspension was administered by oral gavage. Thirty minutes after receiving the charcoal meal, rats were euthanized and the small intestine was removed. The length of the small intestine and the distance traveled by the charcoal were recorded. For each animal, gastrointestinal transit was calculated as the percentage of the distance traveled relative to the total length of the small intestine.

RESULTS

Baseline (vehicle dosed animals) gastrointestinal transit was significantly greater in fasted versus fed rats. In fasted rats, morphine did not have a significant effect on transit. In fed rats, 25 and 75 mg/kg morphine caused a significant decrease in transit. In fed and fasted rats, 0.5 and 3 mg/kg clonidine caused a significant decrease in transit. Loperamide did not affect gastrointestinal transit in fed or fasted rats at doses up to 40 mg/kg.

DISCUSSION

These data demonstrate that food does not reduce the sensitivity of the gastrointestinal transit time.

Origins, practices and future of safety pharmacology

The origins of safety pharmacology are grounded upon observations that organ functions (like organ structures) can be toxicological targets in humans exposed to novel therapeutic agents, and that drug effects on organ functions (unlike organ structures) are not readily detected by standard toxicological testing. Safety pharmacology is " em leader those studies that investigate the potential undesirable pharmacodynamic effects of a substance on physiological functions in relationship to exposure in the therapeutic range and above em leader " [International Conference on Harmonization (ICH) S7A guidelines; Safety Pharmacology Studies for Human Pharmaceuticals]. This publication provides a comprehensive review of the history of safety pharmacology, international regulatory guidelines that govern the practices of this important field, and the scientific challenges that are being faced by its rapid emergence in pharmaceutical development. The criticality of identifying undesired adverse effects of new drugs in nonclinical models, which reflect the overall human condition, is reflected in the importance of generating an integrated and accurate assessment of possible human risk. The conundrum posed by the challenge of formulating a reliable risk assessment is the importance of improving and enhancing the safe progression of new drugs to the marketplace, while preventing unnecessary delays (or discontinuances), based on nonclinical findings that are not relevant or interpretable in terms of clinical response or human risk.