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Factors that influence the proliferation of marrow elements can be detected in sera. To determine the function and to compare the effect of these factors, cells were obtained from patients with normal and leukemic bone marrows. The effects of drug-induced stimulatory and inhibitory sera and leukemic pretreatment sera over time (0 to 6 days) on proliferation and granulocytic morphology of normal and leukemic bone marrow cells in culture were evaluated. Increased proliferation was associated with stimulatory sera, while inhibitory and leukemic pretreatment sera retarded proliferation of both normal and malignant cells. Exposure of normal proliferative cells to inhibitory or leukemic pretreatment sera yielded the greatest increase in mature granulocyte forms. In contrast, although the proliferative response of leukemic cells to these sera was similar to normal, maturation was minimal. These data suggest that leukemic pretreatment sera are similar to inhibitory sera and are not leukemogenic. Both retard proliferation of normal and leukemic bone marrow cells while enhancing maturation of normal cells. Leukemic myeloblasts, however, cannot be made to mature by these humoral regulators.

To define the relationship between perturbed cell growth and intracellular metabolism of 1-beta-D-arabinofuranosylcytosine (ara-C) in sensitive human cells, growth kinetic, and biochemical pharmacological determinants were examined in normal human bone marrow populations in vitro in normal serum and in the presence of drug-induced humoral stimulatory activity (HSA). Cells cultured in HSA demonstrated both increased proliferation and greater ara-C-related inhibition of DNA synthesis than did cells maintained in normal serum, as measured by [3H]thymidine incorporation into DNA and [3H]thymidine granulocyte precursor labeling index. Parallel measurements of [3H]ara-C incorporation into DNA demonstrated similar behavior in HSA-perturbed cells. When these cultured cells were exposed to 1 and 10 microM ara-C, intracellular formation of 1-beta-D-arabinofuranosylcytosine 5'-triphosphate over 3 hr and retention of this active form during 1 subsequent hr in drug-free medium were both increased in HSA-stimulated cells relative to cells cultured in normal serum. These studies demonstrate coupling of induced cell growth kinetics with enhanced intracellular metabolism of the S-phase-specific antimetabolite ara-C in normal human marrow cells. The close direct relationship between growth kinetic perturbation and augmentation of intracellular ara-C activation in this normal hematopoietic model provides a basis for comparison with leukemic cell populations, in which uncoupling of growth kinetics and pharmacokinetics may signify divergence from normal drug-sensitive cell behavior and, thus, resistance to ara-C cytotoxicity.

Clinical trials with laboratory correlates were conducted in patients with acute leukemia to determine if relationships exist between drug dose and growth of surviving leukemia cells. The therapeutic design was based on findings in the leukemic rat that relate the initial dose of drug and tumor kill to the magnitude of residual tumor proliferation and sensitivity to a second drug. Patients with acute myelocytic leukemia received cytarabine, either 2 or 6 g/m2/72 h by continuous infusion. The presence and magnitude of change between initial and residual tumor after treatment, as measured by change in labeling indices, depended on the "priming" dose of drug. The amount of perturbation correlated with clinical response to cytarabine given at the time of induced proliferation. With results which parallel the rat data, the direct relationship of initial drug dose and proliferation of residual tumor is demonstrated in humans, and lends support to the design of our clinical trials of timed sequential therapy.

To demonstrate that the effect of 1-beta-D-arabinofuranosylcytosine can be increased when the leukemic cell growth fraction is augmented by induced humoral factors, bone marrow cells from 14 patients with acute myeloblastic leukemia were studied in vitro. Cells were cultured in either pooled stimulatory serum, obtained from leukemic patients undergoing chemotherapy, or autologous leukemic pre-treatment serum. After 2 days in culture, serum containing 1-beta-D-arabinofuranosylcytosine was added. Cells cultured in stimulatory serum proliferated markedly when compared with cells cultured in pretreatment serum, as measured by tritiated thymidine incorporation into an acid-insoluble precipitate, tritiated thymidine labeling indices, and viable tumor cell counts. The cytotoxic effect of 1-beta-D-arabinofuranosylcytosine was enhanced in those cells initially cultured in stimulatory serum relative to cells initially cultured in pretreatment serum. These studies are consistent with the hypothesis that induced humoral stimulation of acute myeloblastic leukemic cells in vitro recruits a greater tumor growth fraction and thereby increases the cytotoxicity of cycle-dependent drugs.

To expand on the findings from previous clinical trials that the growth of residual tumor is increased at a predictable time following initial drug administration, malignant plasma cells from bone marrows of patients with multiple myeloma (MM) were examined for changes in proliferation and clonogenicity induced in vivo by cyclophosphamide and in vitro by drug-induced humoral stimulatory activity. Peak plasma cell [3H]thymidine labeling index (LI) occurred predictably following drug and paralleled changes in agar colony formation by marrow cells obtained during therapy. Colony-forming capacity of pretreatment MM marrow populations was enhanced when those cells were cultured with humoral stimulatory activity, similar to the increased colony formation detected in Day 9 postcyclophosphamide marrows at the time of peak plasma cell LI. To further define a relationship between proliferative plasma cells and colony-forming tumor cells, MM marrows were fractionated by sedimentation on an isokinetic gradient. Enrichment of a proliferative tumor cell cohort was achieved, evidenced by [3H]thymidine LI. Colony-forming cells were also enriched by isokinetic gradient sedimentation, and agar colony formation by MM marrow cell fractions correlated with the kinetic characteristics of the isolated subpopulations. These studies of whole and fractionated human MM marrow cell populations suggest that the kinetically active cells which are induced to proliferate in vivo and in vitro are closely related to the clonogenic tumor cells which produce colonies in agar and which, like those cells measured by [3H]thymidine LI, respond to growth stimulation by drug-induced humoral stimulatory activity.

Taxol, a novel antimicrotubule agent that enhances tubulin polymerization and microtubule stability, was administered to adults with refractory leukemias as a 24-h i.v. infusion in a Phase I study. The primary objectives were to determine the maximum tolerated dose of taxol administered on this schedule to patients with acute leukemias and describe the nonhematological toxicities which became dose limiting. The starting dose, 200 mg/m2, was based on the maximum tolerated dose in solid tumor trials, in which myelosuppression precluded dose escalation. Seventeen patients received 28 evaluable courses at 200, 250, 315, and 390 mg/m2. Severe mucositis limited further dose escalation. Other nonhematological effects included peripheral neuropathy, alopecia, myalgias, arthralgias, nausea, vomiting, diarrhea, and an acute pulmonary reaction that was presumptively due to taxol's Cremophor vehicle. Mean peak taxol plasma concentrations at all dose levels were in the range of concentrations that were previously demonstrated to induce microtubule bundles, a morphological effect associated with cytotoxicity, in leukemia cells in vitro. Pretreatment blasts from 12 patients were incubated with taxol ex vivo. Taxol-induced microtubule bundles were apparent in the blasts of eight patients who also had cytoreduction of tumor, and sensitivity to bundle formation was related to the magnitude of antitumor activity. In contrast, taxol did not induce microtubule bundles ex vivo in the blasts of the other four total nonresponders. Based on this study, the maximum tolerated doses and recommended Phase II doses for taxol, limited by nonhematological toxicity and administered as a 24-h i.v. infusion to patients with refractory leukemias, are 390 and 315 mg/m2. Phase II trials at these myelosuppressive doses are required to determine taxol's activity in the treatment of leukemias. In addition, further evaluation of microtubule bundle formation ex vivo in Phase II studies is necessary to determine the ultimate utility of this assay in assessing tumor sensitivity to taxol.