Pharmacotherapy of preterm labor.

Physiological factors initiating the birth process, problems associated with preterm labor, and use of pharmacotherapeutic agents to treat preterm labor are reviewed. Human parturition appears to be initiated by a combination of factors, the interplay of which is not well understood. In many cases, the threat posed by preterm labor is difficult to assess. Because the stringency of patient-selection criteria varies widely among studies, success rates of different drugs used to arrest labor are difficult to compare. For both short-term and long-term tocolysis, beta 2-sympathomimetic agents (betamimetics) can be used. These drugs, which include isoxsuprine, ritodrine, terbutaline, albuterol, nylidrin, fenoterol, metaproterenol, and hexoprenaline, are believed to affect intracellular calcium concentrations in the myometrium. Ritodrine is the only drug in this class currently approved by the FDA for inhibition of labor. Terbutaline has been shown to be effective in halting uterine contractions and is substantially less expensive than ritodrine. Calcium channel-blocking agents such as nifedipine and verapamil are being investigated for inhibition of labor. Magnesium sulfate, another calcium antagonist, has long been used as a tocolytic. Other agents discussed are ethanol, diazoxide, the prostaglandin synthetase inhibitors (e.g., indomethacin and aspirin), and progestational steroids. Pharmacotherapy should be individualized on the basis of the patient's clinical condition, presence of other disease states, and side effects associated with available tocolytic agents. To date, a betamimetic with selective beta 2 effects, such as terbutaline or ritodrine, is the most valuable agent for inhibition of preterm labor.