Bio-pharmaceutics is the branch of pharmaceutical science which deals with the formulation factors (chemical nature of drug, additives and processes to manufacture dosage form) that influence the bioavailability of drug and eventually its biological effects.
It can also be defined as the study of relationship of the physicochemical properties and in vitro behavior of the drug after its delivery into the body. The drugs are formulated rationally based on pharmaceutics properties. Biopharmaceutical principles are applied to determine the dose and dosage form of a drug product, which would enhance the delivery of active drug and optimize the therapeutic efficacy of the drug.
Main concern in bio pharmaceutics is the bioavailability of drug. The biological effects are proportional to the amount of drug made available to the body i.e. bioavailability. The effectiveness of dosage form is proportional to bioavailability. In other words, bio pharmaceutics deals with factors influencing the bioavailability.
If the two formulations produce equal blood levels, they are considered as bioequivalent or equally bioavailable. Two drug products containing equal doses of a drug are said to be bioequivalent, if they do not differ significantly in either their bioavailable dose or its rate of supply.
In bio pharmaceutics, the rate of drug released from the product and the rate of drug absorption are important in determining the distribution of the drug in the body. Both biopharmaceutical and kinetic principles are applied in the design of dosage forms.
Various terms used in bio pharmaceutics for better understanding are:
(i) Drug product:
Finished dosage form that contains the active drug ingredient.
(ii) Pharmaceutical equivalents:
Drug products that contain identical amounts of the same active drug ingredient i.e., the same salt or ester of the same therapeutic moiety – in identical dosage form.
(iii) Pharmaceutical alternatives:
Drug products that contain an identical therapeutic moiety, or its precursor, but not necessarily in the same amount or dosage form or as the same salt or ester.
(iv) Bioequivalent Drug Products:
Pharmaceutical equivalents whose rate and extent of absorption may not show a significant difference when administered at the same dose under similar experimental conditions, either in single dose or in multiple dose.
Biopharmaceutical studies are conducted to develop a dosage form that will make the drug bioavailable at a consistent rate. Drug concentrations higher than the therapeutic window may cause more intense pharmacodynamics/and or toxic response; drug concentrations below the therapeutic window may be sub-therapeutic.
A consistent bioavailability of drug is important when the drug has a narrow therapeutic index as a higher bioavailability may lead to toxicity and a slightly lower bioavailability may result in lower concentration. Bio-pharmaceutics is closely related to pharmacokinetics as it involves the rates of drug transfer which employs kinetic methods.
Knowledge of the pharmacokinetic profile of the drug is important for estimation of amount (dose) of drug in the drug product and the rate of release that will maintain a desired drug level in the body. The therapeutic window determines the desired or target plasma drug concentration that will be effective with minimal adverse effects.
For drugs with a narrow therapeutic window the pharmacokinetic profile enhances drug therapy for many products through the development of an appropriate dosage regimen including the size of the dose and the dosing frequency that will achieve and maintain the target drug concentration.
Biopharmaceutical considerations often determines the ultimate dose and dosage form of a drug product. The dose is generally based on the body weight or surface area, to account for the differences in the apparent volume of distribution. Thus, dose is expressed as total mass (mg or g) in terms of body weight (mg/kg) or on the body surface area (mg/m2).
(v) Dose and Dosing Regimen:
The size of the dose determines the plasma drug concentration required for the desired therapeutic effect. For some drugs, wide variation in the size of the dose is needed due to large interspecies/inter-subject differences in the pharmacokinetics and bioavability of the drug.
Drug products are manufactured in different dosing strength. The size and shape of the drug product is determined by the dose of the drug. The drug product is available as tablets, boluses, powders, capsules, suppositories, injections etc. In veterinary practice, injectable formulation is preferred over oral administration due to complication of swallowing and large stomach.
The dosing frequency depends on the clearance of the drug and target plasma drug concentration. If the drug has short elimination half-life and rapid clearance from the body, the drug must be given at short interval. In order to avoid repeated administration, a sustained release drug product is preferred. A sustained release product contains two or more doses of the drug released over a prolonged period of time.