Civie Teddy takes it for intestinal lymphoma. He has been on it for 6+ years, in combination with a steroid (formerly Prednisolone 5 mg, now sub Q Dexamethasone). When I last consulted with his oncology vet, she said the leukeran/pred combo is still the gold standard for intestinal lymphoma. He has been on two different dosing strategies. He was initially on a pulse chlorambucil (leukeran) schedule which involved a higher dose (4 mg) once daily for 4 days, repeated every 3 weeks. He later moved to a maintence schedule of a lower dose (2 mg) every second day.
Leukeran is also prescribed as treatment for IBD. I tried it for Bear Man, but it was not successful for him. IBD and intestinal lymphoma are sort of on a continuum of disease. Have you discussed doing some biopsies via colonoscopy? They are not as diagnostic as the full thickness biopsies taken with an open incision, but they can yield some useful information. However, since you have started treatment with steroids, there is no point in doing any biopsies now - the results would be affected by the steroid treatment. He would have to go off treatment for about 6 weeks before considering biopsies.
Leukeran may cause some side effects such as nausea, vomiting, and inappetance. Teddy also takes Ondansetron to combat the slight nausea. Without it, he had a poorer appetite on chemo days. Other drug therapies which are important in IBD/lymphoma disease are Metronidazole (an antibiotic which also has anti-inflammatory properties on the gut), Pepcid, and B12 (cyanocobalamin) injections. You could consider an appy stim if he isn't eating well. Teddy also takes Ursodiol for his pancreatitis. The steroid and Leukeran combo (or either one alone) will suppress the immune system, so you should watch out for things that might pose a threat, such as exposure to upper respiratory viruses.
Since Teddy has been in remission from his cancer for so many years, I am a huge fan of Leukeran. The tablets should be refrigerated, and you should handle them with gloves.