Injectable Drug Bioavailability

Injectable Drug Bioavailability

Injectable drug bioavailability refers to the ability of a drug to be absorbed into the bloodstream and achieve the desired therapeutic concentration. The rate and extent of drug absorption depend on various factors, especially the injection site.

Injection Site and Its Impact on Bioavailability:

• Subcutaneous (SC): Absorption is slower than intramuscular (IM) due to lower blood flow.

• Intramuscular (IM): Absorption is faster than subcutaneous due to higher blood flow.

• Intravenous (IV): Absorption is the fastest. The drug is directly introduced into the bloodstream, achieving therapeutic concentrations rapidly (within 5 minutes) with 100% bioavailability.

Types of IV Injections:

• Bolus injection: A single dose is injected intravenously, and blood drug concentrations decline rapidly after 30 minutes.

• Infusion: The drug is slowly infused intravenously, maintaining a stable blood concentration for an extended period, depending on the drug and indication.

• Direct injection into the pericardium, pain blocking injections: These injections have immediate effects and are used in emergency situations.

Impact of Excipients and Drug Solubility:

• Drugs with large molecules: (e.g., albumin, dextran, insulin, procaine penicillin G) are absorbed slowly and eliminated slowly, leading to prolonged effects.

• Polymer excipients: (e.g., Na CMC, PEG, PVP, polysorbate) help bind the active substance, causing slow metabolism and are used in drugs with controlled release.

• Controlled-release drugs: (e.g., tramcinolone acetonide, hydrocortisone) can control bioavailability based on the solubility of polymer excipients in water or oil.

• Drugs in oil-based solvents: Absorption is slower than with water-based solvents.

Impact of Isotonicity:

• Isotonic injections: Good compatibility with living cells, better tolerated than hypotonic and hypertonic injections.

• Slightly hypertonic injections: Fastest absorption rate.

Note: The bioavailability of injectable drugs follows general rules of absorption, diffusion, and elimination. Choosing the appropriate injection site, drug form, and excipients helps to adjust bioavailability and achieve optimal therapeutic outcomes.