• A larger dose is the most obvious way to prolong a drug action. As this is not always feasible, other mechanisms are used.
• Vasoconstriction will reduce local blood flow so that distribution of drug away from an injection site is retarded, e.g. local anaesthetic action is prolonged by combination with adrenaline (epinephrine).
• Slowing of metabolism may usefully extend drug action, as when a dopa decarboxylase inhibitor, e.g. carbidopa, is combined with levodopa (as co-careldopa) for parkinsonism.
• Delayed excretion is seldom practicable, the only important example being the use of probenecid to block renal tubular excretion of penicillin, e.g. when the latter is used in single dose to treat gonorrhoea.
• Molecular structure may be altered to prolong effect, e.g. the various benzodiazepines.
• Pharmaceutical formulation. Manipulating the formulation in which a drug is presented by modified-release25 systems can achieve the objective of an even as well as a prolonged effect.
24 For example: Livingston EH, Lee S 2001 Body surface area prediction in normal-weight and obese patients. American Journal of Physiology Endocrinology and Metabolism 281: 586-591
25 The term modified covers several drug delivery systems. Delayed-release: available other than immediately after administration (mesalazine in the colon); sustained-release: slow release as governed by the delivery system (iron, potassium); controlled-release: at a constant rate to maintain unvarying plasma concentration (nitrate, hormone replacement therapy).
Sustained-release (oral) preparations can reduce the frequency of medication to once a day, and compliance is made easier for the patient. Most long-term medication for the elderly can now be given as a single morning dose. In addition sustained-release preparations may avoid local bowel toxicity due to high local concentrations, e.g. ulceration of the small intestine with potassium chloride tablets, and may also avoid the toxic peak plasma concentrations that can occur when dissolution of the formulation, and so absorption of the drug, are rapid. Some sustained-release formulations also contain an immediate-release component to provide rapid, as well as sustained, effect.
Depot (injectable) preparations are more reliable because the environment in which they are deposited is more constant than can ever be the case in the alimentary tract and medication can be given at longer intervals, even weeks. In general such preparations are pharmaceutical variants, e.g. micro-crystals, or the original drug in oil, wax, gelatin or synthetic media. They include phenothiazine neuroleptics, the various insulins and penicillins, preparations of vasopressin, and medroxyprogesterone (i.m., s.c.). Tablets of hormones are sometimes implanted subcutaneously. The advantages of infrequent administration and better patient compliance in a variety of situations are obvious.
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