The hydrochloride salt of a guanylhydrazone with potential antineoplastic activity. Mitoguazone competitively inhibits S-adenosyl-L-methionine decarboxylase (SAMD), an enzyme involved in the synthesis of polyamines, resulting in decreased proliferation of tumor cells, antimitochondrial effects, and p53-independent apoptosis. Polyamines, specifically spermine and spermidine, are essential for thymidine kinase production, DNA synthesis, and cell proliferation.