<prompt> Xenobiotics are metabolized to ?Increase water solubilityIncrease lipid solubilityMake them nonpolarNone of the above  <response>: Ans. is a i.e., Increase water solubility BIOTRANSFORMATION METABOLISM Most of the drugs are treated by the body as foreign substances xenobiotics. Like other foreign substances xenobiotics, body tries to eliminate drugs by various mechanisms for ridding itself of chemical intruders. Biotransformation means chemical alteration of the drug in the body. Why drug transformation is necessary ? Kidney plays a pivotal role in terminating the activity of drugs. For renal excretion the drug tends to be polar lipid insolublewater soluble so that it can not diffuse back from tubular lumen and can be excreted. But pharmacologically active organic molecules drugs tend to be lipophlic nonpolar and remains unionized or only paially ionized at physiological pH. Biotransformation is needed to render nonpolar lipid soluble compounds polar water soluble so that they are not reabsorbed in the renal tubules and are excreted. Sites and processes of biotransformation Primary site of drug metabolism is liver, others are - kidney, intestine, lung and plasma. Biotransformation of drugs may lead to :- Active metabolite from an active drug Many drugs are paially conveed to one or more active metabolites. The effects observed are the sumtotal of that due to the parent drug and its active metabolite. Activation of inactive drugs Few drugs are inactive as such and need conversion in the body to one or more active metabolites. Such a drug is called prodrug. 