The major alkylating agents are the nitrogen mustards and the nitrosoureas. These drugs covalently alkylate various cellular constituents. Most importantly for cancer treatment, alkylation occurs between the bases of DNA molecules of rapidly dividing cancer cells. This reaction cross-links the bases of DNA, causing cessation of DNA synthesis and cell death. The most significant effect is to bind and cross-link double-stranded DNA; therefore, these drugs are referred to as bifunctional alkylating agents. Bifunctional alkylating agents are more cytotoxic and produce fewer drug-induced tumours than monofunctional agents. Alkylation of the DNA molecule causes abnormal base pairing, misreading of the genetic code and excision of bases, which prevents DNA transcription and RNA synthesis.
These drugs are more active on growing cells in the cell cycle than on dormant ceils. However, they can act at any point of the cell cycle and therefore are non-cycle-specific. They are most active when DNA is dividing, such as in the G1 Phase and S Phase. As a consequence, in addition to their effect on cancer cells, they will also affect rapidly growing normal cells such as bone marrow cells and gastrointestinal mucosa.
The nitrogen mustards (bischloroethylamines) are a group of bifunctional alkylating agents that alkylate various macromolecules, but preferentially alkylate N-7 of the guanine base of DNA. They are cytotoxic to cancer cells and are toxic to the rapidly dividing cells of the bone marrow.
Cyclophosphamide: Cyclophosphamide is probably the most potent of the nitrogen mustards. It is used in chemotherapy protocols for a variety of tumours: carcinomas; sarcomas; feline lymphoproliferative diseases; mast cell tumour; mammary carcinoma; and, especially, lymphoproliferative tumours (lymphoma). The main indication for the nervous system is treatment of central nervous system lymphoma and as an immunosuppressive agent in immune-mediated diseases. A typical dose is 50 mg/m2 orally 4 days per week. Pulse doses may be as high as 200 mg/m2 once a week to once every 21 days.
Cyclophosphamide must be metabolized to active metabolites for pharmacological effect and some of the activation requires a P-450 enzyme activation; other steps in the activation are non-enzymatic. The metabolites, hydroxyphosphamide and aldophos-phamide, are cytotoxic. Aldophosphamide is converted at the tissue site to phosphoramide mustard and acrolein, which are responsible for its biological activity (i.e. alkylating activity and cytotoxicity). The half-life of the parent drug in dogs is 4-6.5 hours.
Toxic effects: In many protocols, cyclophosphamide is administered chronically at relatively low oral doses rather than large pulse doses. With low doses acute toxic effects are not as common. Nevertheless, with cyclophosphamide therapy, bone marrow suppression, gastrointestinal toxicosis and cystitis are important concerns. Cyclophosphamide is toxic to the bone marrow in a dose-dependent manner. After a large pulse dose, maximum toxicity occurs in 7-10 days but the effect is reversible because the stem cells are spared. Recovery usually occurs in 21-28 days. The nadir of bone marrow activity appears to be delayed in cats and it takes longer for the myelosuppression to resolve.
Toxicosis to the gastrointestinal tract can occur because the cytotoxic products of metabolism affect the rapidly dividing cells of the gastrointestinal mucosa. Nausea and vomiting may occur as a consequence of acute therapy. Sterile haemorrhagic cystitis is a serious complication to therapy that may require abrupt termination of therapy. It is caused by the toxic effects of metabolites on the bladder epithelium (especially acrolein) that are concentrated and excreted in the urine.
Various attempts have been made to decrease the injury to the bladder epithelium. Corticosteroids are usually administered with cyclophosphamide to induce polyuria and decrease inflammation of the bladder. The drug mesna (mercaptoethanesulfonate) provides free active thiol groups to bind metabolites of cyclophosphamide in the urine, decreasing haemorrhagic cystitis. Mesna is a thiol compound oxidized in the blood to a disulphide dimesna. It is taken up by kidneys and excreted in urine as mesna. In the urine, it combines with acrolein and forms inert non-toxic metabolites. This complex does not injure the epithelial cells of the bladder. The activity of mesna is limited to the urine and therefore it does not affect anti-tumour effect of cyclophosphamide. Cats are less susceptible than dogs to developing cystitis.
Another adverse effect is alopecia. This effect is reversible and is related to hair follicle toxicity. It is primarily seen in dogs with continuously growing hair (e.g. Poodles, Old English Sheepdogs). Cats do not tend to lose hair from cyclophosphamide treatment.
Immunosuppressiveproperties: Cyclophosphamide is frequently used for immune-mediated disorders in small animals. The effect of cyclophosphamide on these diseases is related to the cytotoxic effects on lymphocytes, including B cells. Mechanisms of im-munosuppression include depletion of lymphocytes, suppression of B cell function, and suppression of cell-mediated immunity by T cells. Cyclophosphamide also suppresses neutrophil and macrophage function.
The nitrosoureas have received the most attention for treating tumours of the brain, in particular gliomas. These drugs are more lipophilic and penetrate the blood-brain barrier betterthan the other anticancer drugs. Two nitrosoureas are commonly used:
- • Lomustine (1-(2-chloroethyl)-3-cyclohexyl-1-chloroethylnitrosourea); known by the abbreviation CCNU
- • Carmustine (1,3-bis-2-chloroethyl-1-nitrosourea); known by the abbreviation BCNU.
These drugs, in addition to being lipid-soluble, are alkylating agents. Both of the nitrosoureas are metabolized spontaneously to alkylating and carbamoylating compounds. The binding occurs preferentially at the 0-6 of guanine. Bifunctional interstrand cross-links are responsible for the cytotoxicity of nitrosoureas. Oral absorption and high membrane penetration are attributed to high lipophilicity. Because oral absorption is high, these drugs can be administered effectively as tablets rather than an injection. After absorption, lomustine is metabolized to anti-tumour metabolites. Both the parent drug and the metabolites are lipid-soluble. The central nervous system penetration of lomustine has been estimated from the plasma:CSF ratio, which is 1:3.
Clinical use: Lomustine has been used more often than carmustine. It has been administered to small animals at doses of 70 mg/m2 to 90 mg/m2 orally every 4 weeks. For brain tumours, protocols of 60-80 mg/m2 orally every 6-8 weeks have also been cited (Fulton and Steinberg, 1990). These protocols are quite different from those for humans, where a dose of as much as 150-200 mg/m2 is recommended.
Adverse effects: The adverse effects of lomustine are primarily attributable to the bone marrow effects. In humans the time to nadir of bone marrow activity can be as long as 4-6 weeks, with slow recovery rates. In dogs maximal bone marrow effects are generally seen 6-7 days after dosing. The doses cited above have been used to minimize the bone marrow effects. At higher doses (e.g. 100 mg/m2) myelosuppression has been reported. Thrombocytopenia as a cumulative effect has also been reported from lomustine administration.
Nitrosoureas can be toxic to the rapidly dividing cells of mucosa. In humans, nitrosoureas have caused pulmonary fibrosis and hepatotoxicosis. In one report 6.1% of 179 treated dogs developed hepatotoxicity. The doses administered were 50-110 mg/m2, at a minimum of every 3 weeks. Signs of hepatic injury were delayed for a median duration of 11 weeks and may be related to cumulative dose. The hepatic damage may be irreversible in dogs. In humans carmustine has been associated with a higher rate of hepatic injury than lomustine, butthere are currently no reports of carmustine administration being used for treating tumours in dogs. In cats, lomustine has been used for treating tumours but there is no record of treatment for central nervous system tumours. Lomustine has been used at a dose of 50-60 mg/m2 orally every 5-6 weeks. In cats maximum bone marrow toxicity occurs at 3-4 weeks.