A metabolite is the intermediate end product of metabolism. The term metabolite is usually restricted to small molecules. A drug metabolite is a byproduct of break down, or “metabolizing,” a drug into a different substance.

Estimation of Drug metabolites in the blood is an essential part to understand the distribution of drug in the blood. High purity, well characterized drug metabolite standards are required for Bio-Analytical BE/BA Studies.

Some metabolites stay in the body much longer than the parent drug. When that is the case, a drug test has a higher probability of identifying a drug user by looking for the metabolites of the drug, rather than the parent drug.

• A metabolite is the intermediate end product of metabolism. The term metabolite is usually restricted to small molecules. A drug metabolite is a by-product of break down, or "metabolizing," a drug into a different substance. 
• Estimation of Drug metabolites in the blood is an essential part to understand the distribution of drug in the blood. High purity, well characterized drug metabolite standards are required for Bio-Analytical BE/BA Studies. 
• Identification and characterization of metabolites is a critical step in the long and expensive process of bringing a new chemical entity to market. For drug candidates, every significant metabolite must be identified, and sufficient quantities synthesized to meet MIST requirements. 
• Metabolite identification plays key roles in drug discovery, pre-clinical development, and clinical development. Metabolite ID studies are designed not only to screen metabolites these purpose-driven assays can also answer different questions. As the compounds progress through different stages, the goals and details of metabolite identification also change. 
• Some metabolites stay in the body much longer than the parent drug. When that is the case, a drug test has a higher probability of identifying a drug user by looking for the metabolites of the drug, rather than the parent drug. 
• If the metabolite half-life is longer than the half-life of the parent drug, it is important to ensure that the sampling duration allows for full characterization of the metabolite’s kinetics. 
• Pharmacologically active metabolites can contribute significantly to the overall therapeutic and adverse effects of drugs. Therefore, to fully understand the mechanism of action of drugs, it is important to recognize the role of active metabolites. 
• Metabolite profiling in context of drug development aims at identifying and quantifying metabolites including their intermediates of an investigational product (IP). Such data are considered of critical value for understanding the routes of elimination, predicting drug–drug interactions, and anticipating safety concerns in humans. 
• Most recently, it has been encouraged by the Food and Drug Administration (FDA) agency to initiate metabolite profiling in humans as early as possible during drug development. This is primarily driven by the interest to prevent unexpected safety issues that may arise from disproportionate (i.e. human-specific) toxic metabolites potentially leading to program termination eventually at a late st age of drug development. Hence, an early understanding of the metabolite profile in humans may increase drug safety which is of greatest regulatory and public health interest. In addition, such knowledge may contribute to reduced attrition rates and overall development costs. 
• In terms of efficacy, metabolite profiling could also facilitate describing the time course of drug effects in case of active metabolites. This is particularly interesting in case one of the active metabolites may have favourable properties over the parent compound. Such positively differentiating metabolite may then be subjected to further clinical development as stand-alone product which can be a valuable ‘by-product’ arising from metabolite profiling. A number of marketed drugs such as cetirizine, fexofenadine, or desloratadine actually represent active metabolites of a parent compound with less favourable properties 
• Metabolite profiling allows estimating the relative importance of drug-metabolizing enzymes/pathways for the elimination of the IP which is of relevance for concomitant drug use requirements, need for drug–drug interaction studies, and pharmacokinetic (PK)-based differentiation aspects. 
• Two technologies are being applied for metabolite profiling purposes, namely nuclear magnetic resonance spectroscopy (NMR) and mass spectroscopy (MS) [ 4 ]. NMR-based metabolite profiling is best suited for comprehensive, structural identification of metabolites, while MS-based approaches allow for metabolite quantification with high sensitivity provided availability of synthesized metabolite standards. Recent advancements in bio analytical technology allow integration of metabolite profiling in first-in-human (FIH) or phase-0 studies using micro-tracers. 
• Metabolites are biomolecules, micro as well as macro molecules are either intermediates or products of metabolism. 

They are usually restricted to small molecule. They can be 

• Primary metabolites 
• Secondary metabolites

• Primary metabolites:  metabolites which are directly involved in body’ s metabolic reactions like normal growth, development , reproduction etc. 
• They have definite role and function in metabolism and are essential for growth and development. 
• Examples: alcohol, amino acids, nucleic acids, vitamins etc. 
• Secondary metabolites : organic compounds synthesised by plants ,fungus,and microbes not as essential part of metabolic pathway but as side products 
• Not directly involved in growth and development .play role in ecological functions like defence medicinal use flavourings etc. 
• Examples: Pigments like anthocyanin, alkaloids like morphine, terpenoides, toxicins, antibiotics drugs like vinblastine, polymeric substances like rubber and gums.