Induction of leukemia in rat by pulse doses of 7,12-dimethylbenz(a)anthracene.

Under simple conditions, an intravenous injection (pulse dose) of large but tolerable amounts of certain polynuclear aromatic hydrocarbons evokes mammary cancer in every rat,' whereas leukemia arises rarely. In the present work, it was found that by rearrangement of the experimental conditions, leukosis and leukemia were induced in high yield but with small incidence of cancer of the breast. In rat, a pulse dose of 7,12-dimethylbenz(a)anthracene damages only those cells which synthesize DNA,2 whereas cells which proliferate by meiosis are not injured.3 4 A single pulse dose of 7,12-DMBA effectively induces mammary cancer under stated conditions. By definition, an effective amount of 7,12DMBA is that quantity which induces mammary cancer in every rat but is not lethal to any animal. Three equal fractions of the largest tolerable and effective amount given at intervals of 3 days called forth more cancers than the total amount injected on one occasion;5 each of the fractions itself evoked cancers in every animal. The leukemogenic action of hydrocarbons was discovered by Morton and Mider' by repeatedly painting the skin of mice with 3-methylcholanthrene. The incidence7 of leukemia was 99 per cent with mean latent period of 86 days; in the predominant type of leukemia there was enlargement of thymus, liver, spleen, and lymph nodes. In the rat, prolonged feeding of aromatic hydrocarbons elicited leukemia in rather low yields. The incidence of leukemia evoked by various hydrocarbons in feeding experiments was 3-MC (ref. 8), 14 per cent; 7,12-DMBA (ref. 9), 47 per cent; 2-acetylaminophenanthrene (ref. 10), 48 per cent. Experimental.-Normal rats of Long-Evans strain, bred at random, were used exclusively. They were provided a commercial ration and water ad libitum and kept in air-conditioned rooms at 250 ± 1. A lipide emulsion" of 7,12-DMBA (0.5% w/w) was injected in a caudal vein. No single injection exceeded 6 mg of 7,12-DMBA. The day of the first pulse dose is designated day 0. The rats were weighed thrice weekly. Leucocyte count, differential cell count, and hemoglobin were determined in heparinized venous blood. Components of the buffy coat were measured in a slender glass capillary tube (75 mm in length; 0.3 mm internal diameter; wall thickness 0.2 mm) which was filled with blood and centrifuged at 12,000 rpm for 5 min; the thickness of the layers of the buffy coat was measured immediately by microscopy using an ocular micrometer; the results are expressed in percentage of the length of this microhematocrit. Surgical operations were performed under ether anesthesia. Prior to the first pulse dose of 7,12-DMBA, in some experiments, spleen was exteriorized to a subcutaneous position, leaving its vascular pedicle intact. Biopsy of liver was done through an abdominal incision, 1 cm in length, and a wedge (ca. 50 mg) of the periphery of the organ was excised for histologic study.