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In the drug development process, the prospective drugs undergo a number of trials and are screened at various stages to generate the final potent drug for the treatment of various diseases. During the screening process, various properties are tested to see if the drug is suitable for the therapeutics and is non-toxic to the living system. After going through the different stages, the candidate found most suitable for the purpose is selected. Formulation of the drug plays a very important role in drug delivery to the target area within the body.

The testing of the viability of the drug within the living system is a very crucial issue in the drug discovery process. The drug must remain viable within the living system until it produces the required effect within the body after reaching the target organ or system. If the drug is very potent for the treatment but is immediately degraded after its entry into the living system due to various enzymes and the physical or biochemical environment within the living system, the administration of such a drug serves no use. Hence, the degradation and thereby clearance of the drug from the system must be avoided for proper drug action to take place. In such cases, drug formulation becomes important. Formulation of a drug has become a very important step in the drug discovery and development process. The potent drugs or biopharmaceuticals have different modes of delivery into the biological system like oral, pulmonary, nasal, transmucosal or transdermal routes depending on the characteristics of the drug, the drug target as well as the advantages of using the specific delivery route.

Formulation plays an essential role in the proper absorption of drugs within the living system. Proper formulation is necessary in solubilizing the drug in the formulation medium such that it helps in the delivery and release of the drugs to the target. Thus, formulation helps in increasing the bioavailability of the drug within the system by increasing its solubility, thus helping in improving its pharmacokinetics. The increase in the bioavailability helps in increasing the therapeutic effect of the drug shortening the time of the production of favourable effect and also in reducing the frequency of dose of the drug. It can also reduce the side effects of the drugs and the tissue specific formulations can help in the reduction of toxicity due to a particular formulation.

Different types of formulations are devised depending on the drug delivery route like for e.g. emulsion or solution for intravenous; solid, suspension or solution for oral; emulsion, suspension or solution for subcutaneous, etc. In case of insoluble compounds, various strategies have been adopted for proper formulation like:

1. the adjustment of the pH of the solution in case of ionizing compounds
as ionization increases the solubility of the compounds;

2. use of co-solvents like glycerine, ethanol, PEG, DMSO, etc to increase
its solubility, though proper care must be taken in the choice of co-
solvent in case of animal PK studies due to possible toxicity from the
co-solvent;

3. Surfactants like Tween 80, SDS, polysorbate, etc in proper dilutions
are used to help in the micellization of the drugs within the body,
which enhances the drug solubility, prevents precipitation due to
surface properties of the drugs, prevents aggregation of protein-based
drugs, thereby enhancing the stability of the drugs within the system.

Lipid based formulations are devised for the lipophilic compounds that are delivered as emulsion. The technology of the liposome delivery system has progressed in the application of oncology as well as for the treatment of viral, bacterial, and fungal infections. Its versatility is the reason, which has helped in the formulation of many classes of drugs.

The choice of proper formulation is very crucial in the PK studies, toxicity studies and in animal pharmacology studies. In the pharmacokinetic (PK) screening of the drug candidates, solubility is an important parameter and the choice of proper formulation is essential as the PK profile must remain unaffected while increasing the solubility of the drugs. The choice of proper surfactant and co-solvent is necessary such that it does not cause any form of toxicity while using the formulated drug. Optimal formulation is very important in the development of animal PK studies to demonstrate drug efficacy and activity. Recent studies have shown that formulation is being studied seriously in gene-delivery systems and in the delivery of drugs using nanoparticles.