Intramuscular Administration

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More than 50% of hospitalized patients receive intramuscular drug administration. The popularity of this route is due to the decreased hazard of administration when compared with the intravenous route. Large volumes of solution can be injected (2-10 mL) by this route, generally with less pain and irritation than is encountered with the subcutaneous route.

Aqueous solution of drugs are usually absorbed from intramuscular administration sites within 10 to 30 minutes, but faster or slower absorption rates are possible depending on the vascularity of the site (blood flow rates range from 0.02 to 0.07 mL/min), the ionization and lipid solubility of the drug, the volume of injection, the osmolality of the solution, and other variables, including coadministered drugs and adjuvants in the formulation.

The small molecules are absorbed directly into the capillaries from the intramuscular administration sites, whereas large molecules gain access to the circulation by way of the lymphatic channels.

The drugs which are poorly water-soluble, such as digoxin and diazepam, or those drugs which dissolve at pH values far above the physiologic range are often administered in nonaqueous media such as propylene glycol or in strongly acid or alkaline aqueous solutions. However, after intramuscular administration these drugs may not stay in solution, resulting in slow or incomplete absorption. In some instances the total bioavailability may be less than that from oral administration, as is demonstrated with phenytoin, diazepam and cefamandol.

The high lipid-soluble molecules are quickly absorbed from intramuscular administration sites, whereas lipid-insoluble molecules diffuse between interstitial fluid and plasma only through the pores in the capillary membrane; this is generally not the rate-limiting step in the absorption. Only very large lipid-insoluble molecules which must be absorbed through the lymphatic system have a rate limitation in their absorption, due to the slow rate of lymph flow (0.1% of the plasma flow).

The concentration of the injected solution can also affect the rate of absorption. For example, atropine is absorbed more rapidly administered in a smaller volume of more concentrated solution. Absorption rates can be accelerated by spreading the solution over large tissue areas, e.g., by massaging or using high-pressure injection devices.

The blood flow to the administration site is often the rate-limiting step in the absorption of drugs. Absorption is more rapid after injection into the deltoid than into the vastus lateralis, and is slowest after gluteal muscle injection. The drugs can be absorbed faster after administration into a buttock in males due to greater adipose tissue in females. Absorption rates increase during exercise regardless of the site of intramuscular administration since this results in increased blood flow to skeletal muscles. Conversely, absorption rates decrease in circulatory shocks, hypotension, congestive heart failure, myxedema, and other disturbances of the circulatory system.

Absorption rates can often be quite erratic upon intramuscular administration of drugs. This is due to increased membrane contact as the solution spreads, change in drug concentration as a result of absorption, a possible hypertonic effect drawing water to the site, or to the precipitation of the drugs. The precipitation can lead to incomplete absorption due to extremely slow redissolution or to phagocytosis of the drug particles. Examples of these incompletely absorbed drugs are ampicillin, cephaloridin, cephradine, phenytoin, and quini-dine. Conversely, the slow absorption of drugs can itself be exploited to produce prolonged administration. The slow absorption can be accomplished by the use of injection vehicles of high viscosity, such as glycerin, cottonseed oil, sesame oil, or polyethylene glycols. Another technique involves preparation of fatty acid ester derivatives, such as decanoate derivative of

TABLE 7 Drugs That Can Undergo Biotransformation in the Lumen or During Absorption in the Mucosa

Acetylsalicylic acid

Meperidine

Aldosterone

Methadone

Aminobenzoic acid

a-Methyl dopa

Aminohippuric acid

Nitrates, organic

Chlorpromazine

Pentazocine

Cortisone

Progesterone

Dexamethasone

Propoxyphene

I-Dopa

Salicylamide

Estrogens

Stilbesterol

Hippuric acid

Sulfonamides

Hydrocortisone

Testosterone

Isoproterenol Terbutaline

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