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The uptake of radiolabeled CLZ and CCNU by L1210 leukemia and murine bone marrow cells was investigated to determine whether the preferential ratio of alkylation of L1210 DNA to murine bone marrow DNA of 1.3 by 0.1 mM CLZ, as against a ratio of 0.6 by equimolar CCNU, is secondary to differences in uptake. The concentration of intact CLZ was determined in the medium and the intracellular water space. The cell: medium ratio (intracellular concentration/medium concentration) of CLZ in bone marrow cells was greater than that seen for L1210 cells. However, the intracellular CLZ concentration generally remained constant in both cell types at 37 degrees C, between 7.0 and 10.0 pmole/microliters. The L1210: murine bone marrow cell ratio of intracellular CLZ concentrations was approximately 1.0 from 10 to 60 min. The intracellular CCNU concentration during the uptake of 0.1 mM (chloroethyl-U-14C) CCNU at 37 degrees C was constant at 85 pmol/microliters from 10 to 60 min in L1210 cells, but slowly decreased from 66 pmole/microliters at 20 min to 43 pmole/microliters at 60 min in bone marrow cells. The L1210: murine bone marrow cell ratio of intracellular CCNU concentrations ranged from 1.45 to 1.98 from 20 to 60 min. Thus, it appears that the preferential ratio of alkylation of L1210 DNA to murine bone marrow DNA by CLZ compared with equimolar CCNU cannot be explained by differences in uptake of the two agents by the two cell types. The uptake of 0.1 mM CLZ at 37 degrees C by L1210 cells in McCoy's 5A medium containing 300 mg% glucose was not affected by the addition of 5 mM cold drug, nor was it affected by the absence of glucose in the medium, with or without cold drug. This suggests that CLZ uptake into L1210 cells is via passive diffusion and that CLZ does not enter these cells via the glucose transport mechanism.

As determined by a colorimetric assay measuring parent compounds plus ether-extractable nitroso-containing metabolites, N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) disappeared more rapidly than N-(2-chloroethyl)-N'cyclohexyl-N-nitrosourea (CCNU) and N-(2-chloroethyl)-N'-(4-transmethylcyclohexyl)-N-nitrosourea (MeCCNU) and their products from the tissues of mice injected IV. Assay of selected samples by high-pressure liquid chromatography revealed that the colorimetric assay for BCNU was specific in that the two assays gave equivalent results. Following IV-injections of CCNU and MeCCNU, however, levels of the parent compounds decreased much more rapidly than the total, color-producing material. Of seven tissues, the largest Cxt values for BCNU, as determined by the colorimetric assay, were noted for blood (442 microgram-min/ml) and large intestine (285 microgram-min/g). Liver (29 microgram-min/g) had the lowest Cxt value, reflecting rapid metabolism of the compound by this organ. Color-producing material related to CCNU disappeared from the solid tissues of mice in a manner generally parallel to that for blood. Of the Cxt values for this compound and its products, those for lung (1753 microgram-equivalents-min/g), kidney (1633 microgram-equivalents-min/g), and small intestine (1557 microgram-equivalents-min/g) were highest. Consistent with its slower rate of metabolism, MeCCNU and its color-producing metabolites remained in most tissues of mice for 12 h following injection. Except for brain (1434 microgram-equivalents-min/g), Cxt values for this nitrosourea and its metabolites in tissues were higher than those of blood (5485 microgram-equivalents-min/ml), with kidney (15,324 micrograms-equivalents-min/g), liver (12,921 microgram-equivalents-min/g), and large intestine (11,501 microgram-equivalents-min/g) being highest. For each nitrosourea, a fair correlation was observed between the Cxt values for tissues and the toxic and/or antitumor effects at those sites.

To obtain a clearer definition of the relationship between the structure of BCNU and CCNU and their antitumor activity, we determined the uptake, distribution, and tissue/plasma partition ratios of both compounds in normal organs and intracerebral (ic) 9L tumors in rats. Greater uptake, distribution, and tissue/plasma partition ratios were obtained for parent CCNU in fat, liver, and brain, and for parent BCNU in kidney. CCNU distributes more rapidly and extensively than BCNU only in fatty tissues. BCNU distributed more extensively in kidney and liver. Rats received bolus IV injections of 14C-labeled BCNU or CCNU in increasing doses; measurements taken 30 min after injection showed that three- to fourfold more BCNU than CCNU was bound to nucleic acids in brain and ic 9L tumor tissue. Because the chloroethyl group is the alkylating moiety for both drugs, these findings implied that BCNU biotransformed to its reactive intermediate more rapidly than did CCNU. These observations, together with previous findings, indicate that one reason for the greater effectiveness of BCNU than CCNU against ic 9L tumors is its superior ability to form an intermediate that can bind to ic 9L tumor cell nucleic acids.