Anticancer drugs: effect on the cloning of Raji lymphoma cells in soft agar

The effect of 11 anticancer drugs on the ability of Raji lymphoma cells to form colonies in soft agar was determined with the use of both a 1-hour and a continuous drug exposure. Three distinct patterns of drug sensitivities were observed: a) Dactinomycin, adriamycin, bleomycin, mitomycin C, vincristine, and cis-platinum II all produced a dose-dependent reduction in colony formation following a 1-hour exposure, which was further augmented by a continuous exposure to the drugs; b) the antimetabolites (methotrexate, beta-cytosine arabinoside, and 5-fluorouracil) and pentamethylmelamine had no suppressive effects on colony formation with a 1-hour exposure, but they produced marked cytotoxicity with continuous drug exposure; and c) L-phenylalanine mustard had the same degree of colony suppression with both a 1-hour and a continuous drug exposure. Preincubation of Raji cells with an enzyme mixture (DNase + pronase + collagenase) did not alter the degree of colony suppression observed with the anticancer drugs. These results indicate that continuous drug exposure should be compared to a 1-hour drug incubation to determine in vitro drug sensitivities of fresh human tumors in the soft agar clonogenic assay, because the 1-hour drug exposure may not identify certain drugs that are potentially clinically active.