Drug Interactions between Prempro and somatropin

MONITOR: Coadministration with inducers of CYP450 3A4 may decrease the plasma concentrations and pharmacologic effects of medroxyprogesterone, which is primarily metabolized by the isoenzyme. Aminoglutethimide, a CYP450 3A4 inducer, has been shown to significantly decrease the serum levels of medroxyprogesterone by 50% or more when administered at 250 mg two to four times daily to women with breast cancer receiving high-dose medroxyprogesterone orally. The decrease was accompanied by an increase in serum cortisol level, which suggests diminished adrenal suppressive effect of medroxyprogesterone. The interaction has not been studied with depot formulations of medroxyprogesterone. Because the clearance of medroxyprogesterone is approximately equal to the rate of hepatic blood flow, drugs that induce CYP450 3A4 are not expected to significantly affect the pharmacokinetics of medroxyprogesterone administered parenterally. In one study, no interaction was reported when medroxyprogesterone was administered intravenously with aminoglutethimide.

MANAGEMENT: Pharmacologic response to medroxyprogesterone should be monitored more closely whenever a CYP450 3A4 inducer is added to or withdrawn from therapy, and the dosage adjusted as necessary. When administered as the depot formulation for contraception, no dosage adjustment for medroxyprogesterone is currently recommended during coadministration with CYP450 3A4 inducers. However, consideration may be given to decreasing the dosing interval (e.g., from one injection every 12 weeks to every 10 weeks) if an interaction is suspected.

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MONITOR: Coadministration of oral estrogen replacement therapy may diminish the therapeutic effects of human growth hormone. Some of the pharmacodynamic effects of human growth hormone are primarily mediated by insulin-like growth factor 1 (IGF-1), and orally administered estrogen can suppress growth hormone (GH)-dependent IGF-1 production in the liver. Gender differences have been shown in responsiveness to GH, with women requiring higher doses than men to achieve a similar response. The results of one study demonstrated that women taking oral estrogen required higher doses of GH to achieve an adequate response compared with men or women using transdermal estrogen. In addition, the study showed that GH requirements were also higher for GH-deficient women receiving primarily oral estrogen replacement therapy than for eugonadal GH-deficient women. This interaction does not appear to apply to non-orally administered estrogens.

MANAGEMENT: Patients receiving human growth hormone with oral estrogen replacement therapy should be monitored for potentially diminished GH efficacy. A higher dosage of growth hormone may be required to maintain serum IGF-1 levels within the normal age-appropriate range in order to achieve the defined treatment goal. Please consult individual product labeling for the appropriate initial dosage of growth hormone recommended. Dosage adjustments as well as clinical and laboratory monitoring should be considered following the addition or discontinuation of oral estrogen replacement therapy during use of growth hormone.

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