Cyclophosphamide

Cyclophosphamide (INN, trade names Endoxan, Cytoxan, Neosar, Procytox, Revimmune), also known as cytophosphane, is a nitrogen mustard alkylating agent from the oxazaphosphinans group.

An alkylating agent adds an alkyl group (CnH2n+1) to DNA. It attaches the alkyl group to the guanine base of DNA, at the number 7 nitrogen atom of the imidazole ring. This interferes with DNA replication by forming intrastrand and interstrand DNA crosslinks.

Cyclophosphamide is used to treat cancers and autoimmune disorders. As a prodrug, it is converted by liver cytochrome P450 (CYP) enzymes to form the metabolite 4-hydroxy cyclophosphamide that has chemotherapeutic activity.

Cyclophosphamide has severe and life-threatening adverse effects, including acute myeloid leukemia, bladder cancer, hemorrhagic cystitis, and permanent infertility, especially at higher doses. For autoimmune diseases, doctors often substitute less-toxic methotrexate or azathioprine after an acute crisis.

It is on the World Health Organization's List of Essential Medicines, a list of the most important medication needed in a basic health system.

Medical uses
Cyclophosphamide is used for the treatment of numerous malignant processes and certain autoimmune diseases. Goals of therapy are prompt control of the underlying pathological process and discontinuation or replacement of cyclophosphamide with less toxic, alternative medication as soon as possible in order to minimize associated morbidity. Regular and frequent laboratory evaluations are required to monitor renal function, avoid drug-induced bladder complications, and screen for bone marrow toxicity.

Like other alkylating agents, cyclophosphamide is teratogenic and contraindicated in pregnant women (Pregnancy Category D) except for life-threatening circumstances in the mother. Additional relative contraindications to the use of cyclophosphamide include lactation, active infection, neutropenia, or bladder toxicity.

Cancer
The main use of cyclophosphamide is with other chemotherapy agents in the treatment of lymphomas, some forms of brain cancer, leukemia and some solid tumors. It is a chemotherapy drug that works by inducing the death of certain T cells.

A 2004 study showed the biological actions of cyclophosphamide are dose-dependent. At higher doses, it is associated with increased cytotoxicity and immunosuppression, while at low, continuous doses, it shows immunostimulatory and antiangiogenic properties. A 2009 study of 17 patients with docetaxel-resistant metastatic hormone refractory prostate cancer showed a prostate-specific antigen (PSA) decrease in 9 of the 17 patients. Median survival was 24 months for the entire group, and 60 months for those with a PSA response. The study concluded low-dose cyclophosphamide "might be a viable alternative" treatment for docetaxel-resistant MHRPC and "is an interesting candidate for combination therapies, e.g., immunotherapy, tyrosine kinase inhibitors, and antiangiogenisis."

Autoimmune diseases
Cyclophosphamide decreases the immune system's response, and although concerns about toxicity restrict its use to patients with severe disease, it remains an important treatment for life-threatening manifestations of autoimmune diseases where disease-modifying antirheumatic drugs (DMARDs) have been ineffective. For example, systemic lupus erythematosus with severe lupus nephritis may respond to pulsed cyclophosphamide. Cyclophosphamide is also used to treat minimal change disease, severe rheumatoid arthritis, Wegener's granulomatosis and multiple sclerosis.

Side effects
Adverse drug reactions from cyclophosphamide are related to the cumulative medication dose and include chemotherapy-induced nausea and vomiting, bone marrow suppression, stomach ache, hemorrhagic cystitis, diarrhea, darkening of the skin/nails, alopecia (hair loss) or thinning of hair, changes in color and texture of the hair, and lethargy. Other side effects may include easy bruising/bleeding, joint pain, mouth sores, slow-healing existing wounds, unusual decrease in the amount of urine, or unusual tiredness or weakness.

Cyclophosphamide is itself carcinogenic and may increase the risk of developing lymphomas, leukemia, skin cancer, transitional cell carcinoma of the bladder or other malignancies. Myeloproliferative neoplasms, including acute leukemia, non-Hodgkin lymphoma, and multiple myeloma, occurred in 5 of 119 rheumatoid arthritis patients within the first decade after receiving cyclophosphamide, compared with one case of chronic lymphocytic leukemia in 119 rheumatoid arthritis patients without a history of cyclophosphamide use. Secondary acute myeloid leukemia (therapy-related AML, or "t-AML") is thought to occur either by cyclophosphamide inducing mutations or selecting for a high-risk myeloid clone. This risk may be dependent on dose and a number of other factors, including the condition being treated, other agents or treatment modalities used (including radiotherapy), treatment intensity and length of treatment. For some regimens, it is a very rare occurrence. For instance, CMF-therapy for breast cancer (where the cumulative dose is typically less than 20 grams of cyclophosphamide) seems to carry an AML risk of less than 1/2000th, with some studies even finding no increased risk compared to the background population. Other treatment regimens involving higher doses may carry risks of 1-2% or higher, depending on regimen. Cyclophosphamide-induced AML, when it happens, typically presents some years after treatment, with incidence peaking around 3–9 years. After nine years, the risk has fallen to the level of the regular population. When AML occurs, it is often preceded by a myelodysplastic syndrome phase, before developing into overt acute leukemia. Cyclophosphamide-induced leukemia will often involve complex cytogenetics, which carries a worse prognosis than de novo AML.

Acrolein is toxic to the bladder epithelium and can lead to hemorrhagic cystitis, which is associated with microscopic or gross hematuria and occasionally dysuria. Risks of hemorrhagic cystitis can be minimized with adequate fluid intake, avoidance of nighttime dosage, and mesna (sodium 2-mercaptoethane sulfonate), a sulfhydryl donor which binds and detoxifies acrolein. Intermittent dosing of cyclophosphamide decreases cumulative drug dose, reduces bladder exposure to acrolein, and has equal efficacy to daily treatment in the management of lupus nephritis.

Cyclophosphamide has also been found to significantly increase the risk of premature menopause in females and of infertility in males and females alike, the likelihood of which increases with cumulative drug dose and increasing patient age. Such infertility is usually temporary but can rarely be permanent. The use of leuprolide in women of reproductive age before administration of intermittently dosed cyclophosphamide may diminish the risks of premature menopause and infertility.

Cyclophosphamide is a Pregnancy Category D drug and has been shown to cause birth defects. First trimester exposure to cyclophosphamide for the treatment of cancer or lupus has shown a pattern of anomalies labeled "cyclophosphamide embryopathy," including growth restriction, ear and facial abnormalities, absence of digits, and hypoplastic limbs. Women previously treated with alkylating agents are often able to conceive and deliver health children.

Neutropenia or lymphoma arising secondary to cyclophosphamide usage can predispose patients to a variety of bacterial, fungal and opportunistic infections. There are no published guidelines for PCP prophylaxis for patients with rheumatological diseases receiving immunosuppressive drugs, but some advocate its use when receiving high-dose medication.

Pulmonary injury appears rare, but can present with two distinct clinical patterns: an early, acute pneumonitis and a chronic, progressive fibrosis. Cardiotoxicity is a major problem with oncology patients treated with higher dose regimens.

High-dose intravenous cyclophosphamide can also cause the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and a potentially fatal hyponatremia when compounded by the intravenous fluids administered to prevent drug-induced cystitis. While SIADH has been described primarily with higher doses of cyclophosphamide, it can also occur with the lower doses used in the management of inflammatory disorders.

Pharmacology
Oral cyclophosphamide is rapidly absorbed and then converted by mixed-function oxidase enzymes (cytochrome P450 system) in the liver to active metabolites. The main active metabolite is 4-hydroxycyclophosphamide, which exists in equilibrium with its tautomer, aldophosphamide. Most of the aldophosphamide is then oxidised by the enzyme aldehyde dehydrogenase (ALDH) to make carboxycyclophosphamide. A small proportion of aldophosphamide freely diffuses into cells, where it is decomposed into two compounds, phosphoramide mustard and acrolein. The active metabolites of cyclophosphamide are highly protein bound and distributed to all tissues, are assumed to cross the placenta and are known to be present in breast milk. Cyclophosphamide metabolites are primarily excreted in the urine unchanged, and drug dosing should be appropriately adjusted in the setting of renal dysfunction. Drugs altering hepatic microsomal enzyme activity (e.g., alcohol, barbiturates, rifampin, or phenytoin) may result in accelerated metabolism of cyclophosphamide into its active metabolites, increasing both pharmacologic and toxic effects of the drug; alternatively, drugs that inhibit hepatic microsomal enzymes (e.g. corticosteroids, tricyclic antidepressants, or allopurinol) result in slower conversion of cyclophosphamide into its metabolites and consequently reduced therapeutic and toxic effects.

Cyclophosphamide reduces plasma pseudocholinesterase activity and may result in prolonged neuromuscular blockade when administered concurrently with succinylcholine. Tricyclic antidepressants and other anticholinergic agents can result in delayed bladder emptying and prolonged bladder exposure to acrolein.

Mechanism of action
The main effect of cyclophosphamide is due to its metabolite phosphoramide mustard. This metabolite is only formed in cells that have low levels of ALDH. Phosphoramide mustard forms DNA crosslinks both between and within DNA strands at guanine N-7 positions (known as interstrand and intrastrand crosslinkages, respectively). This is irreversible and leads to cell apoptosis.

Cyclophosphamide has relatively little typical chemotherapy toxicity as ALDHs are present in relatively large concentrations in bone marrow stem cells, liver and intestinal epithelium. ALDHs protect these actively proliferating tissues against toxic effects of phosphoramide mustard and acrolein by converting aldophosphamide to carboxyphosphamide that does not give rise to the toxic metabolites phosphoramide mustard and acrolein.

Cyclophosphamide induces beneficial immunomodulatory effects in adaptive immunotherapy. Suggested mechanisms include:


 * 1) Elimination of T regulatory cells (CD4+CD25+ T cells) in naive and tumor-bearing hosts
 * 2) Induction of T cell growth factors, such as type I IFNs, and/or
 * 3) Enhanced grafting of adoptively transferred, tumor-reactive effector T cells by the creation of an immunologic space niche.

Thus, cyclophosphamide preconditioning of recipient hosts (for donor T cells) has been used to enhance immunity in naïve hosts, and to enhance adoptive T cell immunotherapy regimens, as well as active vaccination strategies, inducing objective antitumor immunity.

History
As reported by O.M. Colvin in his study of the development of cyclophosphamide and its clinical applications,

"Phosphoramide mustard, one of the principle toxic metabolites of cyclophosphamide, was synthesized and reported by Friedman and Seligman in 1954 …It was postulated that the presence of the phosphate bond to the nitrogen atom could inactivate the nitrogen mustard moiety, but the phosphate bond would be cleaved in gastric cancers and other tumors which had a high phosphamidase content. However, in studies carried out after the clinical efficacy of cyclophosphamide was demonstrated, phosphoramide mustard proved to be cytotoxic in vitro (footnote omitted), but to have a low therapeutic index in vivo."

Cyclophosphamide and the related nitrogen mustard-derived alkylating agent ifosfamide were developed by Norbert Brock and ASTA (now Baxter Oncology). Brock and his team synthesised and screened more than 1,000 candidate oxazaphosphorine compounds. They converted the base nitrogen mustard into a nontoxic "transport form". This transport form was a prodrug, subsequently actively transported into the cancer cells. Once in the cells, the prodrug was enzymatically converted into the active, toxic form. The first clinical trials were published at the end of the 1950s. In 1959 it became the eighth cytotoxic anticancer agent to be approved by the FDA.