Muscles depot injections

Intramuscularly administered products typically form a depot in the muscle mass from which the drug is slowly absorbed. The peak drug concentration is usually seen within 1-2 hours. Factors affecting the drug-release rate from an IM depot include the compactness of the depot (the less compact and more diffuse, the faster the release), the rheology of the product, concentration and particle size of drug in the vehicle, nature of the solvent or vehicle, volume of the injection, tonicity of the product, and physical form of the product. 

The advantages of administration by intramuscular injection are that the muscle can act as a depot, and the rate of disappearance of drug from the site of injection can be calculated. Inhalational, intranasal, and intratracheal administration are normally reserved for vapors and aerosols including anesthetics. Absorption is facilitated by small-sized particles, high lipid solubility, sufficient pulmonary blood flow, and a large absorptive surface area, as it is present in healthy lungs. Administration by these routes can be very rapid when several of the factors favoring increased absorption are combined. 

The depot FGAs fluphenazine decanoate (also available in an enanthate salt) and haloperidol decanoate are esterified drugs formulated in sesame seed oil for deep intramuscular injection. Their absorption from the muscle and metabolism to the free base is sufficiently slow to cause absorption to be the rate-limiting step in determining their respective apparent half-lives. ... 

Intramuscular injections are performed deep into the skeletal muscles at either the deltoid, gluteal, or lumbar muscles. The site is chosen to minimize danger of hitting a nerve or blood vessel. Aqueous or oleaginous solutions or suspensions may be used with rapid effects or depot activity selected to meet the requirements of the patient. Drugs that are irritating to subcutaneous tissue are often administered intramuscularly with volumes of 2 to 5 mL or more. When a volume of 5 mL or more is to be injected it should be in divided doses using two injections. 

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