Arsenicals in hematologic cancers.

Arsenic trioxide (AT) has been the object of renewed interest as a therapeutic since studies in China in the late 1980s confirmed its efficacy in the treatment of acute promyelocytic leukemia (APL). These studies have been replicated in the West, with complete remissions achieved in 80% to 90% of patients with refractory or relapsed APL. The drug has been relatively well tolerated. The dose used for treatment of APL (0.15 mg/kg/d) is approximately 50% of the maximum-tolerated dose (MTD). Common side effects have included fatigue, rash, fluid retention, and QTc-interval prolongation on electrocardiogram. A "retinoic acid syndrome," similar in its manifestations to that noted after administration of all-trans retinoic acid (RA), has been observed in APL patients. Recent studies have included dose-ranging trials to determine pharmacokinetics and the optimum schedule of administration, and studies of possible mechanisms of action. Promising future trials include combining AT with RA in the treatment of newly diagnosed APL, and broadening the range of AT therapy to other leukemias, lymphomas, multiple myeloma and some solid tumors.

Oncogene involvement in tumor regression: H-ras activation in the rabbit keratoacanthoma model.

Activated H-ras genes are present in a number of skin tumors induced in animals by carcinogen treatment. The involvement of the ras oncogenes in tumorigenesis was investigated in keratoacanthomas, benign and self-regressing tumors, as well as malignant squamous cell carcinomas. Both tumors were induced in rabbit ears by repeated applications of 7,12 dimethylbenz(a)anthracene (DMBA). The rabbit H-ras gene was cloned and sequenced. PCR analysis revealed that approximately 82% of the keratoacanthoma DNAs contained an A:T to T:A transversion in codon 61. The relative levels of H-ras transcript were increased in keratoacanthomas compared to normal skin and the activated allele was expressed in tumors, even during the regressing phase. Although a G:C to A:T mutation in codon 12 of the H-ras and an activated N-ras gene were found in two squamous cell carcinomas, the frequency of H-ras activation in codon 61 was much lower (40%) in the malignant tumours induced by the same carcinogen treatment. Therefore, DMBA induced at least two types of genetic lesions in this system: H-ras activation, present in most regressing keratoacanthomas, and activation of other unidentified oncogenes which may result in the development of malignant tumors. Our observations indicate that expression of an activated H-ras gene, in this system, is neither sufficient to induce a malignant phenotype nor even capable of maintaining the growth of a benign tumor and suggest that it could be involved in tumor regression.