Renal impairment and Drug Metabolism

Renal diseases can cause considerable variability in drug response, which may present as enhanced or diminished drug response. Renal diseases (kidney diseases) affect the glomerular blood flow, glomerular filtration, tubular reabsorption and secretion. Renal bioactivation affects the drug metabolism, which causes alterations in drug absorption, bioavailability, distribution, binding of drugs to proteins and renal or non-renal clearance. Accumulation of drug metabolites and exogenous substances may make it necessary for the clinician to adjust the dose of drugs for the renal disease and associated ailments otherwise this variability can lead to either enhanced efficacy or enhanced toxicity depending on the drug characteristics. Renal diseases (renal disorders) may affect both pharmacokinetic and pharmacodynamic characteristics of a drug (as thiopental has been observed to cause prolongation of its anesthetic action and gentamycin to cause increased toxicity).

Dosage adjustment is usually needed if >30% drug is excreted unchanged in urine or is removed by dialysis in patients with renal impairment or renal failure. Impaired and compromised renal function effects the drug elimination and leads to accumulation of drug. The accumulation of drug further depends on the frequency of drug administration and half life of drug. The treating physician needs to know the pharmacokinetics and pattern of accumulation of drugs in renal failure and tentative time to achieve the steady concentration in plasma for the adequate dose adjustments.

Primary influence of renal disease on drug absorption is almost negligible. However, a few secondary influences like vomiting and diarrhoea due to uremia lead to dehydration and further lead to decreased perfusion and absorption of drugs from intramuscular and intestinal sites. Diabetic and uremic gastroparesis leads to altered rates of absorption of short acting drugs like sulfonylureas. Acid base imbalance and alteration in the levels of potassium in serum affects gastrointestinal tract mobility and rate & extent of drug absorption. Commonly used antacids bind and chelate dietary phosphates and lead to hypophosphatemia, also impair the absorption of drugs like iron and ketaconazole which need an acidic pH for optimal absorption. Impaired protein binding may also impair absorption of drugs. Edema caused by nephrotic syndrome or congestive heart failure has been documented to slowdown the drug absorption and cause increased bioavailability of drugs.

Drug Development: Non-Clinical Mutagenicity and Carcinogenicity Studies

The fundamental aim of non-clinical or pre-clinical studies in relation to new chemical entity (NCE) is to generate biological, pharmaceutical and toxicological data to ascertain that NCE would not cause any serious harm to the humans or would not cause cancer and/or genetic disorders through mutations. The limits of toxicological study of a NCE are decided with reference to route and duration of its intended use as a drug. Studies to evaluate carcinogenicity and mutagenicity of a NCE are performed in animals before it is declared as a drug and given to humans. Animal species usually used for carcinogenicity and mutagenicity are rats and mice, as some strains of these animals have very low incidence of spontaneous tumors and sufficient background knowledge about physiological parameters of these animals is available. Guidelines for such studies can be obtained from the regulatory agencies like FDA (USA), DCGI (India) and EMEA (Europe). The NCE is generally administered to the animals by at least two routes, one of which is usually the intended clinical route. The other route is usually intravenous so as to ensure adequate exposure of the animal to the new chemical entity. It is highly desirable to develop sensitive analytical methods for detecting the drug and its metabolites in the body fluids before starting the pre-clinical studies, as this would help in elucidating the toxicokinetics, pharmacokinetics, mutagenicity and carcinogenicity of the NCE. All studies need to be conducted in Good Laboratory Practice (GLP) compliance laboratories.

Carcinogenicity studies are needed to be done in adequate number of animals, depending on the regulatory requirements, before obtaining the approval for marketing of a drug. The drugs or compounds on which mutagenicity, carcinogenicity, teratogenicity and reproductive performance related effects need to be elucidated are of following types:

1. When the drug product is to be used for three months or longer period for at least six months - Carcinogenicity studies are must.
2. When the drug or compound is to be used frequently or in an intermittent manner in chronic or recurrent conditions like depression, anxiety and allergic rhinitis.
3. Prolonged exposure due to certain drug delivery systems makes it mandatory to carry out carcinogenicity studies.

Relaxations and Exemptions:

1. Genotoxic compounds are presumed to be trans-species carcinogens; hence long-term carcinogenicity studies are usually not needed.
2. Pharmaceuticals administered infrequently or for short duration of exposure (such as anesthetics and radiolabelled imaging agents) are exempted from carcinogenicity studies.
3. Carcinogenicity testing need not be conducted before market approval for the drugs developed for treating serious diseases.
4. If the life expectancy in the indicated population is short (less than 3 years), no long-term carcinogenicity studies are not required for the compounds to be administered to these subjects.