Proliferative potential of out-of-cycle leukaemic cells

HERG potassium channels are constitutively expressed in primary human acute myeloid leukemias and regulate cell proliferation of normal and leukemic hemopoietic progenitors

An important target in the understanding of the pathogenesis of acute myeloid leukemias (AML) relies on deciphering the molecular features of normal and leukemic hemopoietic progenitors. In particular, the analysis of the mechanisms involved in the regulation of cell proliferation is decisive for the establishment of new targeted therapies. To gain further insight into this topic we report herein a novel approach by analyzing the role of HERG K(+) channels in the regulation of hemopoietic cell proliferation. These channels, encoded by the human ether-a-gò-gò-related gene (herg), belong to a family of K(+) channels, whose role in oncogenesis has been recently demonstrated. We report here that herg is switched off in normal peripheral blood mononuclear cells (PBMNC) as well as in circulating CD34(+) cells, however, it is rapidly turned on in the latter upon induction of the mitotic cycle. Moreover, hergappears to be constitutively activated in leukemic cell lines as well as in the majority of circulating blasts from primary AML. Evidence is also provided that HERG channel activity regulates cell proliferation in stimulated CD34(+) as well as in blast cells from AML patients. These results open new perspectives on the pathogenetic role of HERG K(+) channels in leukemias.

Modulation of in vitro chemosensitivity in acute myelogenous leukemia cell line by GM-CSF: opposing effects observed with different cytotoxic drugs and time exposure

In some studies the GM-CSF (granulocyte-macrophage colony-stimulating factor) increased the in vitro sensitivity of acute myeloid leukemia (AML) cells to cytosine arabinoside (Ara-C), however, in clinical trials no favorable effects were shown. We used a GM-CSF responsive AML cell line (AML 193) to test the effects of growth stimulation on in vitro efficacy of Ara-C and methotrexate (MTX). In 6 days continuous exposure, dose dependent Ara-C cytotoxicity was counteracted by GM-CSF. Conversely, MTX cytotoxicity was increased significantly. However, in a short term treatment (24 h, high doses) the GM-CSF increased both MTX and Ara-C cytotoxicity. These effects might depend on different drug regimens and cell features.

Addition of etoposide to initial therapy of adult acute lymphoblastic leukemia: a combined clinical and laboratory study

The role of high-dose etoposide in the initial treatment of newly diagnosed adult ALL was assessed in a combined clinical and laboratory study. Therapy on protocol JH8802 consisted of two induction modules, module 1 containing prednisone, vincristine, high-dose etoposide and L-asparaginase (L-asp), followed by module 2 containing cytarabine (Ara-C) and daunorubicin (DNR). Patients achieving a complete remission (CR) underwent bone marrow transplantation (BMT) or intensive maintenance therapy. Results were compared to the preceding protocol (JH8302), which was similar except for omission of etoposide and L-asp. The CR rate following module 1 was 45% on protocol JH8802 and 9% on protocol JH8302 (p < 0.0002). Nonetheless, the two protocols had similar CR rates following module 2 (69% on protocol JH8302; 77% on JH8802) and indistinguishable survivals. Laboratory investigations performed on blasts harvested prior to chemotherapy revealed two factors that could potentially contribute to decreased etoposide sensitivity in ALL blasts. A flow microfluorimetry-based assay of nuclear DNR accumulation detected small P-glycoprotein (Pgp)-mediated decreases in drug accumulation in a quarter of the samples. Western blotting demonstrated that topoisomerase II was present in all samples but was diminished in amount compared to the Molt3 human ALL cell line. Immunoperoxidase staining with affinity-purified antibodies revealed that topo II alpha, the target for etoposide, was detectable in only a minority of the blasts (median 7.5%, range < 1-35%) at diagnosis. These observations raise the possibility that alterations in drug accumulation and diminished target enzyme levels might both limit the long-term efficacy of a single course of high dose etoposide administered early in the treatment of adult ALL.

Different sensitivity of normal and leukaemic progenitor cells to Ara-C and IL-3 combined treatment

In the present study the effects of a combined treatment with cytosine-arabinoside (Ara-C) and interleukin-3 (IL-3) on acute myeloblastic leukaemia clonogenic cells and on normal haemopoietic progenitors was investigated, with the aim of improving the tumoricidal effect of cycle specific drugs. Blast cells from 24 acute myeloblastic leukaemia (AML) patients were screened with a short-term proliferative assay based on 3H-thymidine (3H-TdR) uptake for their response to IL-3. To evaluate the synergism between the growth factor and Ara-C, the cells were pretreated for 3 d in liquid culture in the presence or absence of IL-3 (10 U/ml) and for the last 24 h with Ara-C (3 micrograms/ml). The cells were then washed and seeded in semisolid media to assess their clonogenic ability. The results showed that, in those cases which were good responders to IL-3 in the 3H-TdR uptake assay (19 out of 24), Ara-C exposure eliminated a greater proportion of clonogenic cells if pretreated with IL-3 than if untreated (P less than 0.001), while in cases unresponsive to IL-3 this effect was not significant. Moreover, when the same protocol was applied to bone marrow cells from normal donors, it was found that IL-3 pretreatment did not significantly enhance the toxic effect of Ara-C on day 14 granulocyte-macrophage colony forming units (CFU-GM) and erythroid burst forming units (BFU-E). Finally IL-3 pretreatment was also able to increase the cytotoxic effect of Ara-C on leukaemic cells co-cultured, to simulate clinical AML remission, with normal bone marrow cells. The results indicate that IL-3 may improve the therapeutic index of cycle-specific drugs in AML therapy.

Lymphokine-activated killer (LAK) cells inhibit the clonogenic growth of human leukemic stem cells

The effect of lymphokine-activated killer (LAK) cells on the in vitro clonogenic capacity of acute myeloid leukemia (AML) blasts was investigated in a semisolid medium assay. The leukemic clonogenic capacity of 11 AML cases, selected on the basis of their ability to grow in vitro, was highly reduced following overnight preincubation with LAK effectors. The degree of colony inhibition, which ranged between 66% and 98% (mean 83.8% +/- 11.4 SD), was quantitatively greater than by 51Cr release, which gave rise to lytic values between 5% and 65% (mean 43.2% +/- 19.2 SD). The demonstration that the clonogenic inhibition was still induced following a shorter pre-incubation period (4 hours) suggests that the effect is unlikely to be due only to the generation of cytotoxic activity during the incubation time. The possibility that LAK cells may be employed in the management of residual disease is strengthened by the evidence that the clonogenic potential of samples containing as few as 20% and 14.3% leukemic cells could be almost completely abolished by LAK effectors. These findings further point the possible role of adoptive immunotherapy with interleukin 2/LAK cells in the treatment of patients with acute leukemia.

Interleukin 3 enhances the cytotoxic activity of 1-beta-D-arabinofuranosylcytosine (ara-C) on acute myeloblastic leukaemia (AML) cells

We have evaluated the possibility of enhancing the cell killing effect of ara-C on AML blasts by increasing their proliferative activity with haemopoietic growth factors. Leukaemic cells from 10 AML patients were incubated for 3 d in liquid culture in the presence or in the absence of the human recombinant growth factors IL-1 beta (5 U/ml) and IL-3 (3 U/ml), and subsequently exposed to ara-C (3 micrograms/ml) for the last 24 h. The number of residual leukaemic stem cells was evaluated by a clonogenic assay in semisolid medium. The results showed that ara-C exposure inhibits the proliferation of a higher proportion of clonogenic cells in cultures pretreated with growth factors than in the controls (mean inhibitory values: in the absence of growth factors = 49.8%; with IL-1 beta = 58.3%; with IL-3 78.9%). The effect was statistically significant only when IL-3 was used as a growth factor. The results suggest that haemopoietic growth factors may help to improve the therapeutic index of cytostatic agents.

Proliferation and cell loss of human leukemic cell subpopulations in liquid culture

A kinetic study was performed on leukemic blasts from patients with acute myeloid leukemia, separated into 2 subpopulations by a specific density gradient. The growth curve and the [3H]-thymidine uptake were simultaneously analyzed. While cumulative nucleotide uptake fitted with the growth kinetics in the low-density fraction, such a concordance was not found in the high-density subpopulation. That indicated the occurrence of simultaneous growth and loss in the high density fraction, which could not be evaluated by a simple numerical determination.

Induction of proliferation of acute myeloblastic leukemia (AML) cells with hemopoietic growth factors

Like their normal counterparts, leukemic blasts have recently been shown to respond to hemopoietic growth factors in both suspension culture and in semisolid media. In the present study, we have evaluated the proliferative response of 35 AML cases to colony-stimulating factors (CSFs) containing conditioned media derived from the human cell lines GCT, 5637, MO and MG U87, and to human recombinant IL-1 (rh-IL1), IL-3 (rhIL-3), GM-CSF (rhGM-CSF) and G-CSF (rhG-CSF). In the great majority of cases, an increase of 3H-thymidine (3H-TdR) uptake was obtained in response to at least one conditioned medium. The labeling index (LI) and the growth fraction (GF), evaluated in a restricted group of cases, were also increased by the growth factors, suggesting that they act by recruiting leukemic cells in cycle from the resting compartment. The ability of blast populations to form colonies was also studied. Conditioned media were found to induce or significantly increase the clonogenic capacity in 20 cases out of 22. The response of leukemic cells to human recombinant CSFs and rhIL-1, used alone or in combination, was also assayed. The results, in agreement with those obtained with conditioned media, show that each leukemic case displays a different pattern of response to CSFs, and that optimal growth conditions must be individually assessed. The possibility of increasing the fraction of cycling cells in AML populations may represent a way to render them more sensitive to cytostatic agents, with a view to new therapeutic strategies.

In vivo cell kinetic effects of vincristine on the spontaneous AKR leukemia: recruitment of non-proliferating cells

In order to investigate the influence of a cell-cycle specific agent on the cytokinetic behavior of a leukemic cell population in vivo, labeling studies with tritiated thymidine (3HTDR) followed by administration of vincristine (VCR) were performed on thymic cells of advanced AKR leukemic mice and evaluated utilizing a combined autoradiographic-Feulgen-microspectrophotometric technique. Twelve hours after a single drug injection the stathmokinetic effect of VCR was observed as reflected by an accumulation of cells in the S/G2-M phase of the mitotic cycle. Within 28 h this effect was no longer evident, but the significant increase in % unlabeled S/G2-M cells strongly suggested an influx of previously non-proliferating cells into the proliferating compartment (recruitment).

Clonal growth of leukaemic cells in vitro

Human leukaemic cell specimens were obtained from patients and directly plated into soft agar (t = 0) or cultured for 1 week in liquid phase and then plated in soft agar. Growth for 1 week in liquid phase allowed the clonal growth in agar of leukaemic specimens which were unable to clone at t = 0. Clonal growth after liquid culture consisted of the usual leukaemic type of cluster-colonies, growth of a new type of 'syncytial' cell colony or a mixture of colony types. In addition, marrow from a patient with acute lymphocytic leukaemia produced normal-appearing colonies after 1 week of growth in liquid phase. These studies suggest a similarity in the growth requirements of some leukaemic cells and normal CFUd cells.

Higher frequency of 51--clone in bone marrow mitoses after culture than by a direct method

In a patient with refractory idiopathic sideroblastic anaemia without signs of leukaemia a clone with the karyotype 46,XX,5q--showed markedly different frequencies in mitotic preparations obtained from bone marrow by different methods. This clone was absent or rare (0--15%) in preparations made by a direct method and prevalent (55--95%) after culture for 22 h or 46 h in the presence or absence of methotrexate. In cells without the 51--chromosome there were minor clones with trisomies 8, 14 or 19. These were somewhat more frequent in preparations made by the methotrexate than by the direct method. If these findings are confirmed in larger series of experiments, it will follow that results obtained for different abnormalities and in different disorders by different methods may not be comparable and that modifications of the present cytogenetic methodology for bone marrow may lead to new findings of practical and theoretical significance.

Proliferation kinetics and cyclic AMP as prognostic factors in adult acute leukemia

In 41 adult patients with acute leukemia (myeloblastic, lymphoblastic, and undifferentiated), proliferation kinetics (as determined by double-label autoradiography) and cyclic adenosine 3',5'-monophosphate (cAMP) concentration were studied for their significance in the prediction of responsiveness to cytostatic therapy. Patients with good clinical response had significantly shorter turnover times and higher labeling indices in the bone marrow than did those who failed to respond to treatment. Cases for which cell kinetics did not correlate with clinical response were explained by variance in the distribution of leukemic blasts between the proliferative cell cycle and the resting pool. Good clinical response was also found to be associated with low levels of cAMP in leukemic cells prior to therapy, whereas high cAMP contents predicted failure. Low cAMP concentrations, however, did not necessarily correlate with short turnover times and vice versa. This might be due to fluctuations of the cAMP concentrations during the cell cycle.

Cell cycle kinetics of uninfected and feline leukemia virus-infected canine lymphoma cell lines: effects of methotrexate treatment

The cell cycle kinetics of uninfected and feline leukemia virus-infected canine lymphoma cell lines were determined by autoradiography (PLM method) as follows: DT-5: generation time (TC), 15.2 h; pre-synthetic gap phase (TG1), 3.2 h; DNA-synthetic phase (TS), 8.2 h; post-synthetic gap ph se (TG2), 3.3 h; visible mitotic phase (TM), 0.5 h. 11028: TC, 13.6 h; TG1, 1.9 h; TS, 7.7 h; TG2, 3.4 h; TM, 0.6 h. 11028+FeLV (11028 productively infected with feline leukemia virus): TC, 11.2 h; TG1, 0.2 h; TS, 8.3 h; TG2, 2.1 h; TM, 0.6 h. Exposure of the lymphoma cell lines to methotrexate (MTX) in vitro produces dose-related increases in cellular volume, associated with reductions in cellular proliferation. The relative sensitivities of these cell lines to MTX, measured by the ID50 MTX concentrations for DT-5, 11028, and 11028+FeLV are 118 nM, 122 nM, and 28 nM respectively. The cell kinetic effects of the ID50 MTX concentrations added to cultures of lymphoma cells pulse-labeled with tritiated thymidine are an approximately 2-h prolongation of TC, attributable to a lengthening of TS, with other cell cycle phases not significantly altered. These cell lines are highly tumorigenic when transplanted into the cheek pouches of immunosuppressed hamsters, with inocula of 10(4) cells producing rapidly growing, well vascularized tumors.

New aspects of preleukemic disorders

Preleukemic disorders are a controversial group of panmyelopathic disturbances that often precede the emergence of acute myeloblastic or myelomonocytic leukemia. In most instances, these preleukemic disorders are characterized by slowly developing myeloblastosis of the bone marrow. They include preleukemia, primary acquired panmyelopathy with myeloblastosis or smouldering acute leukemia, erythroleukemia, and subacute myelomonocytic leukemia. Sometimes, transitions between these various preleukemic disorders may be observed in a single individual. Abnormalities in cellular differentiation are expressed in cytochemical aberrations and in elaboration of colony forming units by marrow cells of patients with preleukemic disorders. Cytogenic and cellular kinetic abnormalities link preleukemic disorders closely to acute myeloblastic or myelomonocytic leukemia, although in many patients with preleukemic disorders, conversion to acute leukemia is not observed or perhaps not recognized. Understanding pathogenetic and pathophysiological aspects of preleukemic disorders may shed light on aspects of cellular proliferation and cellular differentiation in the acute leukemias.

Clinical trial of intra-arterial polychemotherapy in the treatment of carcinoma of the oral cavity

Following studies of problems regarding the phenomenon of synchronization and cell recruitment, the authors present the results obtained using antimitotic polychemotherapy (VCR, MTX, BLM, ADM or MTC) administered to 47 patients with squamous cell carcinoma of the oral cavity. This treatment resulted in a regression in 29/47 patients (61.7 %). All patients were subjected to X-ray therapy, while only a part of the responders were subjected to surgery. Survival after 18 months from the beginning of the treatment has been 24/47 patients (51.0%). Since the majority of patients were beyond the limits of surgery and/or radiotherapy, this can be considered as a positive result, even though the risk of death due to polychemotherapy has been rather high.

Analysis of the cycling and noncycling cell population of human solid tumors

The size and kinetics of the cell population of six advanced human solid tumors were analyzed following the continuous infusion of H3-TDR for a period of 6 to 21 days. By using the labelling indices of the cells in interphase and mitosis, and the rate of label incorporation, it was found the only 10 to 40% of the tumor cell population was replicating at any given time. Fifty-five percent to 85% of the remaining cell mass eventually entered into cycle at least once. Anywhere from 5% to 40% of the cell population remained arrested in G2 or G0 in individual cases during the period of observation. The significant size of the "resting" cell compartment is undoubtedly one explanation for the relative insensitivity of advanced adult solid tumors to current cell cycle-oriented therapeutic regimens.

Re-entry of resting leukaemic blood cells into proliferation in human acute leukaemia during diffusion chamber culture

From 17 patients with different forms of acute leukaemia, mononuclear blood cells were cultured in diffusion chambers (DC) implanted intraperitoneally into pre-irradiated mice. In 14 patients, growth of blast cells could be observed during the culture period of up to 21 days. To question whether this growth of blast cells was due only to proliferation of the initially proliferating fraction or whether a re-entry of resting leukaemic cells into proliferation was involved, various 3H-thymidine (3H-TdR) labelling studies were carried out. The absolute increase of blast cells in EC showed no correlation with the fraction of leukaemic blast cells in DNA-synthesis in the implanted cell suspension as measured by 3H-TdR labelling in vitro. Furthermore, in 2 patients where the kinetic behaviour of initially labelled leukaemic blast cells was followed during DC culture, the increase in total blast cells could only be attributed to a small extent to proliferation of those cells initially in the cell cycle. Lastly, "in vivo" labelling during the culture period showed that in one case 25% and in another case 60% of the blast cells in DC were proliferating. The conclusion is that, owing to the stimulation of the diffusion chamber milieu and possibly also due to removal of an in vivo inhibition, in most cases of acute leukaemia resting leukaemic blast cells can apparently re-enter the active cell cycle. This has relevance for an understanding of the self-maintenance of the leukaemic cell population and may also be a reason for relapse of leukaemia after the usual cytostatic drug treatment which affects mainly the proliferating leukaemic cells.

New approach to brain tumour chemoradiotherapy with cellular synchronization by colcemid

Cellular synchronization using Colcemid as pretreatment for combined chemoradiotherapy was investigated. C6 rat brain tumour was cultured in RPMI medium containing 10(-5)-10(-7) Mol. of Colcemid for 24 hours. The basic cell kinetics were analysed with a Pulse Cytophotometer, which facilitated the analysis of tumour cell cycle phase distribution according to the DNA content. The effect of Colcemid depended on the concentration, and the minimal concentration showing continuous blocking during 48 hours after removal of the drug was 10(-6) Mol. G1 fraction of 2 C DNA content was reduced from 74% to 36%. G2-M phase of 4 C DNA content increased from 9% to 28%. S phase cells increased from 17% to 31%. Polyploid cells in the Tetraploid cell cycle could be recognized. The remaining 36% of cells within the GO + G1 peak of 2 C DNA content were considered to be diploid GO cells.

Manipulation of the mitotic cycle in the treatment of acute myelogenous leukaemia

A priming dose of cytosine arabinoside (Ara C) was given to 16 children and five adults with acute myelogenous leukaemia (AML) to synchronize leukaemia cells in the DNA synthesis (S) phase of the mitotic cycle. An infusion of this drug, which was continued for 12 h, was started at the time of partial synchronization and was repeated every 6-12 h until the bone marrow was very hypocellular. Complete remission was achieved in 12 of 16 children and in all adults. Two of the four children went into complete remission with the addition of two doses of daunorubicin or adriamycin. These results suggest that partial synchronization of cells in the S phase results in a therapeutic advanatage in the use of Ara C for induction of a remission of AML and that manipulation of the mitotic cycle, as monitored by kinetic studies, may be helpful in planning optimal schedules for drug administration.

An attempt at synchronization of marrow cells in acute leukemia: relationship to therapeutic response

The relationship between changes in the bone marrow labeling index and the patient's subsequent response to cycle-specific agents was studied by the South-eastern Cancer Study Group in adults with acute leukemia. Ninety-eight patients were randomized to one of two treatment regimens. Schedule 1 consisted of a single intravenous (i.v.) push of cytosine arabinoside followed in 48 hours by a large dose of oral methotrexate distributed over 24 hours and i.v. vincristine. Leucovorin rescue was employed to control the toxic effects of the high dose methotrexate and the cycle was repeated every 7 days. Schedule 2 differed only in that there were three daily injections of cytosine arabinoside preceding vincristine and methotrexate injections and each cycle was given every 10 days. Cell kinetic studies were performed in 30 patients and revealed that the majority of patients who had a response to therapy had some increase in the marrow labeling index 48 hours after cytosine arabinoside injection. In general, those patients who had no response to therapy had little change. There was no significant difference between schedules in the ability to induce an increase in labeling index 48 hours after cytosine arabinoside or in the increment achieved by the responders. However, there was a significant difference in the response rate seen with these schedules. Schedule 1 achieved only a 24% remission rate in acute nonlymphocytic leukemia (ANLL) while schedule 2 was associated with a 52% remission rate. In acute lymphoblastic leukemia (ALL) both schedules induced a 60% remission rate while none of the four patients with blast crisis of chronic granulocytic leukemia (CGL) responded. Analysis of the characteristics associated with remission revealed that more females achieved a remission than males and that the presence of pretreatment infection was the greatest contributing cause of early death and thus severely limited the ability to achieve a remission. As opposed to the current regimens used in ANLL, schedule 2 did not require significant bone marrow hypoplasia (as judged by the degree of hematological toxicity) to achieve a remission and there was no decrease in response seen with increasing age. The data suggest that increased efficiency of cycle-specific, antitumor agents may occur by increasing the proportion of human leukemic cells in DNA synthesis.

Recycling of resting cells in the JB-1 ascites tumour after treatment with 1-beta-D-arabinofuranosylcytosine (ara-C)

Resting cells in tumours present a major problem in cancer chemotherapy. In the plateau phase of grwoth of the murine JB-1 ascites tumour (i.e. 10 days after 2-5 X 10(6) cells i.p.) large fractions of non-cycling cells with G1 and G2 DNA content (Q1 and Q2 cells) are present, and the fate of these resting cells was investigated after treatment with 1-beta-D-arabinofuranosylcytosine (Ara-C). The experimental work of growth curves, percentage of labelled mitoses curves after continuous labelling with 3H-TdR, and cytophotometric determination of single-cell DNA content in unlabelled tumour cells. Treatment with an i.p. single injection of Ara-C 200 mg/kg in the plateau JB-1 tumour resulted in a significant reduction in the number of tumour cells 1 and 2 days later as compared with untreated controls, while no difference in the number of tumour cells was observed after 3 days. In tumours prelabelled with 3H-TdR 24 hr before Ara-C treatment, a significant decrease in the percentage of labelled mitoses was observed 6-8 hr later followed by a return to the initial value after 12 hr, and a new pronounced fall from 20 hr after Ara-C. The second fall in the percentage of labelled mitoses disappeared when the labelling with 3H-TdR was continued also after Ara-C treatment. Cytophotometry of unlabelled tumour cells prelabelled for 24 hr with 3H-TdR before Ara-C treatment showed 20 hr after Ara-C a pronounced decrease in the fraction of Q1 cells paralleled by an increase in the fraction of unlabelled cells with S DNA content. The results indicate recycling of resting cells first with G2 and later with G1 DNA content, which contribute to the regrowth of the tumours.

Mode of action of chemotherapy in vivo on human acute leukemia. I. Daunomycin

The leukocytes of 16 adult patients with acute myeloblastic leukemia were studied by autoradiographic methods to elucidate the mode of action of daunomycin. It was shown that daunomycin, at clinically useful doses, exhibits a cytolytic effect on all leukemic blasts whatever their cell-cycle phase. This cytolytic action affects, however, preferentially S-phase cells. It was shown also that blasts of patients less sensitive to daunomycin or receiving a lesser dose of the drug are temporarily blocked in G(2) phase (delayed mitosis) or in G(2) phase (prolonged generation time). Finally daunomycin appeared to hamper the passage of G(2)-blocked blasts from the bone marrow to the blood, while G(2)-phase cells crossed freely.

Synchronization and recruitment in acute leukemia

The in vivo effects of several chemotherapeutic agents on the mitotic cycle of leukemic blasts in the bone marrow were evaluated by serial measurements of cells in mitosis and in deoxyribonucleic acid (DNA) synthesis as indicated by ability to incorporate tritiated thymidine or tritiated deoxyuridine. 28 studies were done in 23 children and 1 adult. The changes in the marrow after a single injection of L-asparaginase, hydrocortisone, cyclophosphamide, cytosine arabinoside, methotrexate, and an exchange transfusion (62% of the total blood volume) were evaluated. L-asparaginase and hydrocortisone were found to arrest the entry of cells into the S period. Cyclophosphamide appeared to inhibit DNA synthesis, arrest cells in mitosis, and inhibit the entry of cells into the S period. Cytosine arabinoside, and methotrexate inhibited DNA synthesis. During the period of time the cells were inhibited in the S phase by these two drugs, cells continued to enter the S period. Thus partial synchronization was achieved after these two drugs. An exchange transfusion had no consistent effect on the mitotic cycle, but partial synchronization in the S period was seen in one patient. To take advantage of the ability of cystosine arabinoside, to synchronize leukemic cells in the S phase, a second cycle-dependent drug was given at the time the leukemic blasts were synchronized. The second cycle-dependent drugs evaluated were vincristine, methotrexate, and cytosine arabinoside given by intravenous drip over a 12 hr period. Recruitment was found after cytosine arabinoside alone, and after prior synchronization with cytosine arabinoside and then the administration of either of these drugs. The results of these studies indicate that a greater therapeutic advantage can be achieved by a second cycle-dependent drug after synchronization than after the second drug alone.