**4.1. Topical application**

This is the most direct and easiest mode of drug administration. It involves local application of a drug to the site of action e.g. eye drop solutions, sprays and lotions for oral, rectal, vaginal and urethral use. These drugs are absorbed through the cell membranes. Absorption of drugs through the skin is proportional to their lipid solubility since the epidermis behaves like a hydrophilic barrier. Lipid insoluble drugs are therefore suspended in oily vehicles to enhance solubility and hence absorption.

#### **4.2. Oral administration**

**ii.** Pharmacokinetic process; deals with the time course of drug concentration in the

**iii.** The pharmacodynamic process; deals with the mechanism of drug action: that is

**iv.** The therapeutic process; deals with the clinical response arising from the pharmaco‐

**v.** Toxicologic process; deals with adverse effects of drugs arising from either over

Biochemical pharmacology is concerned with the effects of drugs on biochemical pathways underlying the pharmacokinetic and pharmacodynamic processes and the subsequent therapeutic and the toxicological processes. The pharmaceutical process is, however, outside

This depends on the actual biochemical characteristics of the drug and the interaction of drug molecules with body fluids and tissues. The main routes of drug administration are the topical

The route of drug application determines how quickly the drug reaches its site of action. The choice of the route of administration of a drug, therefore, depends on the therapeutic objectives of the treatment. For instance, intravenous injection or inhalation may be selected to produce intense, but rather short-lived effects, whereas oral dosing may be better and more convenient for long lasting effects and even intensity. The various types of drug administration include;

This is the most direct and easiest mode of drug administration. It involves local application of a drug to the site of action e.g. eye drop solutions, sprays and lotions for oral, rectal, vaginal and urethral use. These drugs are absorbed through the cell membranes. Absorption of drugs through the skin is proportional to their lipid solubility since the epidermis behaves like a hydrophilic barrier. Lipid insoluble drugs are therefore suspended in oily vehicles to enhance

dosage or interference of biochemical pathways unrelated to the intended drug

interaction of drugs with the molecular structures in the body.

target. The five processes are related as exemplified in Figure1.

**4. Routes of drug administration and systemic availability**

formation and excretion of the drug.

dynamic process.

476 Drug Discovery

**3. Biochemical pharmacology**

the realms of biochemical pharmacology.

application, parenteral, and enteral routes.

**4.1. Topical application**

solubility and hence absorption.

body.This process can be further subdivided into; absorption, distribution, biotrans‐

The drugs administered orally are absorbed at different sites along the gastrointestinal tract (GIT):

#### *4.2.1. Oral mucosal or sublingual*

Drug absorption is generally rapid because of the rich vascular supply to the mucosa and the ab‐ sence of a stratum corneum. Drugs delivered using this route are not exposed to gastric and in‐ testinal digestive juices and are not subjected to immediate passage through the liver. Therefore there is no prior biotransformation or first-effect before the drugs enter the systemic circulation.

#### *4.2.2. Stomach and intestine*

Absorption depends on different factors such as pH, gastric emptying, intestinal motility and solubility of solid drugs. The rapidity with which a drug reaches the small intestine is enhanced when a drug is taken with water and when the stomach is relatively empty. However drugs absorbed in the stomach and intestine are subjected to first-pass effect.
