Megestrol acetate, a synthetic progestational agent with antineoplastic and appetite-stimulating properties, was developed in the 1960s and introduced into clinical use in the early 1970s. Its history is marked by its initial use in gynecological disorders such as contraception and treatment of endometrial conditions, followed by expanded applications in oncology for hormone-responsive cancers like breast and endometrial carcinoma. Megestrol acetate, a derivative of progesterone, was later found to significantly stimulate appetite and promote weight gain, leading to its use in the management of cancer-associated cachexia and AIDS-related wasting syndrome. Over time, it became an important supportive care medication in chronic illness, with both hormonal and metabolic effects contributing to its therapeutic profile.
BRAND NAMES
Megace ES (extended suspension formulation)
Megecat
Megestrol
MECHANISM OF ACTION
Megestrol acetate is a synthetic progestin that exerts its effects primarily by binding to progesterone receptors, leading to gene transcription changes that regulate hormone-responsive tissues. In hormone-dependent cancers such as breast and endometrial carcinoma, it acts as an anti-estrogenic agent by downregulating gonadotropin secretion (LH and FSH), thereby reducing estrogen production.
PHARMACOKINETICS
Absorption
Megestrol acetate is well absorbed after oral administration, but its bioavailability can vary due to first-pass hepatic metabolism. Peak plasma concentrations are generally reached within 1 to 5 hours after oral intake.
Distribution
Reported values vary, but the apparent volume of distribution is generally in the range of approximately 20–30 L/kg, reflecting significant tissue uptake and storage, especially in fatty tissues. This extensive distribution contributes to its prolonged duration of action.
Metabolism
The primary metabolic pathways involve hydroxylation and conjugation reactions, mainly mediated by hepatic enzymes including the cytochrome P450 system.
Elimination
Megestrol acetate is eliminated primarily after extensive hepatic metabolism, with its inactive metabolites excreted mainly via the urine and, to a lesser extent, in feces. Only a minimal amount of unchanged drug is eliminated.
PHARMACODYNAMICS
Megestrol acetate is a synthetic progestin that exerts its pharmacological effects primarily through activation of progesterone receptors, leading to altered gene transcription in target tissues. In hormone-dependent cancers such as breast and endometrial carcinoma, it suppresses gonadotropin (LH and FSH) release from the pituitary, resulting in decreased estrogen production and reduced stimulation of estrogen-sensitive tumor growth.
ADMINISTRATION
Megestrol acetate is administered orally in the form of tablets or an oral suspension. It is usually taken once daily or in divided doses depending on the indication and prescribed dose. The oral suspension is commonly used for appetite stimulation in cachexia because it allows flexible dosing and easier intake. It may be taken with or without food, although taking it with meals can improve gastrointestinal tolerance.
DOSAGE AND STRENGTH
Megestrol acetate is available in tablet form (20 mg, 40 mg, and 160 mg) and as an oral suspension (commonly 40 mg/mL or 125 mg/mL depending on formulation and country). For appetite stimulation in cancer or AIDS-related cachexia, the usual dose is 400–800 mg per day orally, often given as a single daily dose using the oral suspension for ease of administration.
DRUG INTERACTIONS
Megestrol acetate can interact with drugs that influence hepatic enzyme activity, particularly those affecting the cytochrome P450 system. Enzyme inducers such as rifampicin, phenytoin, and carbamazepine may decrease its plasma concentration and reduce therapeutic effectiveness.
FOOD INTERACTIONS
Megestrol acetate has no clinically significant food restrictions, and it can be taken with or without meals. However, taking it with food may improve gastrointestinal tolerance and reduce the risk of nausea or stomach discomfort.
CONTRAINDICATIONS
Megestrol acetate is contraindicated in patients with known hypersensitivity to megestrol acetate or any of its formulation components. It should not be used as a diagnostic test for pregnancy or in patients with known or suspected pregnancy due to potential fetal harm. Caution is required in patients with a history of thromboembolic disorders, as megestrol may increase the risk of venous thromboembolism.
SIDE EFFECTS
Increased appetite
Weight gain (very common)
Fluid retention (edema)
Nausea
Vomiting
Diarrhea
Abdominal discomfort
OVER DOSE
Megestrol acetate overdose is uncommon, but excessive doses may intensify its pharmacologic and hormonal effects. Symptoms can include severe nausea, vomiting, fluid retention, weight gain, and marked hyperglycemia. Due to its glucocorticoid-like activity, high doses may also lead to adrenal suppression, weakness, and Cushing-like features such as moon face or edema.
TOXICITY
Megestrol acetate toxicity is usually related to prolonged use of high doses rather than acute overdose. Toxic effects are mainly due to its progestational and glucocorticoid-like activity. These may include significant weight gain, fluid retention, edema, hyperglycemia, and Cushing-like features such as fat redistribution and muscle weakness.
