Cyclophosphamide is a cytotoxic alkylating agent developed in the 1950s and introduced into clinical use in the late 1950s as a major advancement in cancer chemotherapy. It is a prodrug that is metabolically activated in the liver to form active metabolites such as phosphoramide mustard, which exert antineoplastic effects by cross-linking DNA and inhibiting rapidly dividing cells. Cyclophosphamide has been widely used in the treatment of various malignancies, including lymphomas, leukemias, breast cancer, and ovarian cancer, and is also utilized in certain autoimmune and immunosuppressive regimens. Its clinical history is marked by strong therapeutic efficacy across multiple disease states, alongside significant dose-limiting toxicities such as bone marrow suppression and hemorrhagic cystitis, which led to the development of preventive strategies like hydration and the use of protective agents such as mesna. Over time, it has remained a cornerstone drug in oncology and immunology due to its broad utility and effectiveness in combination chemotherapy protocols.
BRAND NAMES
Cytoxan – most widely used brand (especially in the US)
Endoxan – commonly used internationally (Europe/Asia)
Neosar – injectable formulation.
MECHANISM OF ACTION
Cyclophosphamide is a prodrug that requires hepatic activation by cytochrome P450 enzymes to form active metabolites, mainly phosphoramide mustard and acrolein. The active metabolite phosphoramide mustard exerts its cytotoxic effect by alkylating DNA, forming intra- and inter-strand cross-links between DNA guanine bases.
PHARMACOKINETICS
Absorption
Cyclophosphamide is well absorbed orally, with high and predictable bioavailability. After oral administration, peak plasma concentrations are typically reached within 1–2 hours. It is also commonly administered intravenously, where absorption is immediate and complete.
Distribution
Cyclophosphamide has a volume of distribution of approximately 0.6–0.7 L/kg, which corresponds to roughly 40–50 liters in an average adult (70 kg).
Metabolism
Cyclophosphamide is extensively metabolized in the liver by cytochrome P450 enzymes (mainly CYP2B6, also CYP3A4 and CYP2C9). It is a prodrug, and hepatic activation converts it into its active metabolites, including 4-hydroxycyclophosphamide, which is in equilibrium with aldophosphamide.
Elimination
Cyclophosphamide is eliminated primarily through renal excretion, with both parent drug and metabolites being excreted in the urine. A significant portion is excreted as inactive metabolites, while a smaller fraction is eliminated unchanged.
PHARMACODYNAMICS
Cyclophosphamide exerts its pharmacological effects through formation of active alkylating metabolites that cross-link DNA strands, particularly at the N7 position of guanine bases. This DNA cross-linking prevents DNA replication and transcription, leading to inhibition of cell division and induction of apoptosis. The drug is most effective against rapidly proliferating cells, including malignant cells and activated immune cells, which explains its use in both cancer chemotherapy and immunosuppressive therapy. Its cytotoxic effect is cell-cycle nonspecific, but it primarily impacts cells with high mitotic activity, resulting in decreased tumor growth and suppression of abnormal immune responses.
ADMINISTRATION
Cyclophosphamide is administered orally and intravenously, depending on the indication and treatment protocol. The oral form is taken as tablets or capsules, usually in once-daily dosing or divided doses, while the intravenous form is given as intermittent infusions in hospital or clinical settings as part of chemotherapy regimens. IV administration allows precise dosing and is commonly used in high-dose or combination cancer therapies.
DOSAGE AND STRENGTH
Cyclophosphamide dosing is highly indication-specific and individualized, based on body surface area (BSA), renal/hepatic function, and treatment protocol. For oral therapy, commonly used tablet strengths include 25 mg and 50 mg.
DRUG INTERACTIONS
Cyclophosphamide has several clinically important drug interactions due to its dependence on hepatic cytochrome P450 metabolism and its potential for significant toxicity. CYP enzyme inducers such as rifampicin, carbamazepine, and phenytoin may increase the formation of active and toxic metabolites, potentially enhancing adverse effects. Conversely, CYP inhibitors like azole antifungals and certain macrolide antibiotics may reduce metabolic activation, decreasing therapeutic efficacy.
FOOD INTERACTIONS
Cyclophosphamide has no major clinically significant food interactions, and its absorption is generally not strongly affected by food intake. However, taking the oral form with food may help reduce gastrointestinal irritation such as nausea and vomiting, although it may slightly delay absorption.
CONTRAINDICATIONS
Cyclophosphamide is contraindicated in patients with known hypersensitivity to cyclophosphamide or any of its components. It should not be used in individuals with severe bone marrow suppression, as it can further worsen cytopenias and increase the risk of life-threatening infections and bleeding.
SIDE EFFECTS
Nausea and vomiting
Loss of appetite (anorexia)
Hair loss (alopecia)
Bone marrow suppression (leukopenia, anemia, thrombocytopenia)
Increased risk of infections
Hemorrhagic cystitis (blood in urine, bladder irritation)
Hematuria
OVER DOSAGE
Overdosage of Cyclophosphamide can lead to severe and potentially life-threateningtoxicity, primarily due to excessive formation of cytotoxic metabolites. Acute overdose typically results in profound bone marrow suppression, leading to severe leukopenia, thrombocytopenia, and anemia, which significantly increases the risk of infections, bleeding, and fatigue. Severe gastrointestinal toxicity such as intractable nausea, vomiting, and mucosal ulceration may also occur.
TOXICITY
Cyclophosphamide toxicity is primarily dose-dependent and related to its cytotoxic metabolites, affecting rapidly dividing normal tissues as well as malignant cells. The most significant toxicity is bone marrow suppression, which can lead to severe leukopenia, anemia, thrombocytopenia, and a high risk of life-threatening infections and bleeding.
