Calicheamicin-conjugated humanized anti-CD33 monoclonal antibody (gemtuzumab zogamicin, CMA-676) shows cytocidal effect on CD33-positive leukemia cell lines, but is inactive on P-glycoprotein-expressing sublines.

Calicheamicin-conjugated humanized anti-CD33 mouse monoclonal antibody, CMA-676, has recently been introduced to clinics as a promising drug to treat patients with acute myeloid leukemia (AML) in relapse. However, the mechanism of action of CMA-676 has not been well elucidated. The cytotoxic effect of CMA-676 on HL60, NOMO-1, NB4, NKM-1, K562, Daudi, and the multidrug-resistant sublines, NOMO-1/ADR and NB4/MDR, was investigated by cell cycle distribution and morphology. These studies were done by a video-microscopic system, DNA fragmentation, dye exclusion and 3H-thymidine uptake after analysis of CD33, CD34, P-glycoprotein (P-gp), multidrug resistance (MDR)-associated protein and lung-related protein on these cells. A dose-dependent, selective cytotoxic effect of CMA-676 was observed in cell lines that expressed CD33, and was dependent on the amount of CD33 and the proliferative speed of the cells. Sensitive cells were temporally arrested at the G2/M phase before undergoing morphological changes. CMA-676 is not effective on P-gp-expressing multidrug-resistant sublines compared with parental cell lines. MDR modifiers, MS209 and PSC833, restored the cytotoxic effect of CMA-676 in P-gp-expressing sublines. CMA-676 is a promising agent in the treatment of patients with AML that expresses CD33. The combined use of CMA-676 and MDR modifiers may increase the selective cytotoxic effect in multidrug-resistant AML.     

Efficacy and tolerability of gemtuzumab ozogamicin (anti-CD33 monoclonal antibody, CMA-676, Mylotarg?) in children with relapsed/refractory myeloid leukemia

Background  

Gemtuzumab ozogamicin (GO) is a cytotoxic anti-CD33 monoclonal antibody that has given promising preliminary results in adult myeloid CD33+ AML. We conducted a retrospective multicenter study of 12 children treated with GO on a compassionate basis (median age 5.5 y). Three patients (2 MDS/AML, 1 JMML) were refractory to first-line treatment, 8 patients with de novo AML were in refractory first relapse, and one patient with de novo AML was in 2^nd relapse after stem cell transplantation (SCT). CD33 expression exceeded 20% in all cases.     

CD33抗体为主方案治疗难治性白血病

目的:评估CD33抗体为主方案治疗难治性白血病的疗效和不良反应。方法:采用CD33抗体为主方案对11例难治性白血病进行治疗。结果:CD33抗体为主方案治疗总有效率54%,其中完全缓解(CR)为36%,除血小板未完全缓解外均达到完全缓解标准(CRP)为18%,治疗后中位生存时间4.8月,缓解后中位生存时间8.2月。2例慢性粒细胞白血病急粒变均完全缓解,且BCR/ABL(-)。1例难治性急性淋巴白血病在缓解后即行异基因干细胞移植,至今无病生存已9月。不良反应有骨髓抑制(100%)、输注相关寒战发热(90%),肝静脉闭塞综合症(VOD)发生率55%,发生在干细胞移植后的患者和年龄大于60岁的患者。结论:本组资料显示CD33抗体为主方案对表达CD33的急慢性白血病有一定疗效。对于移植后复发和年龄大于60岁的患者,在使用CD33抗体时需注意患者肝脏情况,一旦出现VOD,及时处理。对于用药后缓解的患者,应尽早行异基因干细胞移植。     

Efficacy of gemtuzumab ozogamicin on ATRA- and arsenic-resistant acute promyelocytic leukemia (APL) cells.

Acute promyelocytic leukemia (APL) cells express a considerable level of CD33, which is a target of gemtuzumab ozogamicin (GO), and a significantly lower level of P-glycoprotein (P-gp). In this study, we examined whether GO was effective on all-trans retinoic acid (ATRA)- or arsenic trioxide (ATO)-resistant APL cells. Cells used were an APL cell line in which P-gp was undetectable (NB4), ATRA-resistant NB4 (NB4/RA), NB4 and NB4/RA that had been transfected with MDR-1 cDNA (NB4/MDR and NB4/RA/MDR, respectively), ATO-resistant NB4 (NB4/As) and blast cells from eight patients with clinically ATRA-resistant APL including two patients with ATRA- and ATO-resistant APL. The efficacy of GO was analyzed by (3)H-thymidine incorporation, the dye exclusion test and cell cycle distribution. GO suppressed the growth of NB4, NB4/RA and NB4/As cells in a dose-dependent manner. GO increased the percentage of hypodiploid cells significantly in NB4, NB4/RA and NB4/As cells, and by a limited degree in NB4/MDR and NB4/RA/MDR cells. Similar results were obtained using blast cells from the patients with APL. GO is effective against ATRA- or ATO-resistant APL cells that do not express P-gp, and the mechanism of resistance to GO is not related to the mechanism of resistance to ATRA or ATO in APL cells.Leukemia (2005) 19, 1306-1311. doi:10.1038/sj.leu.2403807; published online 26 May 2005.     

Efficacy and safety of gemtuzumab ozogamicin in patients with CD33-positive acute myeloid leukemia in first relapse.

PURPOSE: Three open-label, multicenter trials were conducted to evaluate the efficacy and safety of single-agent Mylotarg (gemtuzumab ozogamicin; CMA-676; Wyeth Laboratories, Philadelphia, PA), an antibody-targeted chemotherapy agent, in patients with CD33-positive acute myeloid leukemia (AML) in untreated first relapse. PATIENTS AND METHODS: The study population comprised 142 patients with AML in first relapse with no history of an antecedent hematologic disorder and a median age of 61 years. All patients received Mylotarg as a 2-hour intravenous infusion, at a dose of 9 mg/m(2), at 2-week intervals for two doses. Patients were evaluated for remission, survival, and treatment-emergent adverse events. RESULTS: Thirty percent of patients treated with Mylotarg obtained remission as characterized by 5% or less blasts in the marrow, recovery of neutrophils to at least 1,500/microL, and RBC and platelet transfusion independence. Although patients treated with Mylotarg had relatively high incidences of myelosuppression, grade 3 or 4 hyperbilirubinemia (23%), and elevated hepatic transaminase levels (17%), the incidences of grade 3 or 4 mucositis (4%) and infections (28%) were relatively low. There was a low incidence of severe nausea and vomiting (11%) and no treatment-related cardiotoxicity, cerebellar toxicity, or alopecia. Many patients received Mylotarg on an outpatient basis (38% and 41% of patients for the first and second doses, respectively). Among the 142 patients, the median total duration of hospitalization was 24 days; 16% of patients required 7 days of hospitalization or less. CONCLUSION: Administration of the antibody-targeted chemotherapy agent Mylotarg to patients with CD33-positive AML in first relapse induces complete remissions with what appears to be a favorable safety profile.     

Final report of the efficacy and safety of gemtuzumab ozogamicin (Mylotarg) in patients with CD33-positive acute myeloid leukemia in first recurrence

BACKGROUND: In this study, the authors analyzed the efficacy and safety of gemtuzumab ozogamicin (GO) (Mylotarg), an antibody-targeted chemotherapy for CD33-positive acute myeloid leukemia (AML). METHODS: Patients with CD33-positive AML in first recurrence were entered in 3 open-label, single-arm, Phase II studies. Patients received monotherapy with GO 9 mg/m(2) as a 2-hour intravenous infusion in 2 doses separated by 2 weeks. Patients were evaluated for remission, survival, and treatment-emergent adverse events. RESULTS: Two hundred seventy-seven patients (median age, 61 yrs) were treated with GO, and 71 patients (26%) achieved remission, which was defined as < or = 5% blasts in the bone marrow without leukemic blasts in the peripheral blood, neutrophil recovery to > or = 1500/microL, hemoglobin > or = 9 g/dL, and independence from red blood cell and platelet transfusions. Complete remission (CR) with platelet recovery (> or = 100,000/microL) or without full platelet recovery (< 100,000/microL) (CRp) was observed in 35 patients (13%) and 36 patients (13%), respectively. The median recurrence-free survival was 6.4 months for patients who achieved CR and 4.5 months for patients who achieved CRp. Although expected incidences of Grade 3 or 4 neutropenia (98%) and thrombocytopenia (99%) were observed, the incidence of Grade 3 or 4 sepsis (17%) and pneumonia (8%) was relatively low. Grade 3 or 4 hyperbilirubinemia and hepatic aspartate aminotransferase and alanine aminotransferase elevations were reported in 29%, 18%, and 9% of patients, respectively; 0.9% of patients who did not undergo prior or subsequent hematopoietic stem cell transplantation developed hepatic venoocclusive disease after GO treatment. CONCLUSIONS: When it was administered to patients with CD33-positive AML in first recurrence, single-agent GO induced a 26% remission rate with a generally acceptable safety profile.     

All-trans retinoic acid (ATRA) differentiates acute promyelocytic leukemia cells independently of P-glycoprotein (P-gp) related multidrug resistance.

Here the relationship between all-trans retinoic acid (ATRA)-resistance and P-glycoprotein (P-gp)-associated multidrug resistance (MDR) is discussed in acute promyelocytic leukemia (APL). First, the remission rates of ATRA therapy are similar in relapsed/refractory APL to the preceding chemotherapy given and in newly diagnosed APL. Second, MDR1 cDNA-transduced NB4 (NB4/MDR) cells accumulate less Rhodamine-123 (Rh123) than NB4 cells, but there is no difference in the intracellular ATRA concentration between them. PSC833 or MS209. MDR modifiers, increases the intracellular accumulation of Rh123 in NB4/MDR and APL cells expressing P-gp, but not of ATRA. Third, the expression of CD11b, the NBT reduction activity, the proportion of apoptotic cells and the morphology are not different between NB4/MDR and NB4 cells, and between APL cells expressing P-gp and not. APL cells express little P-gp, and mainly express CD33 but no CD34. Despite previous reports that ATRA-resistant APL cells express more P-gp than ATRA-sensitive ones, P-gp and ATRA-resistance seems to exist independently.     

CD33-directed therapy with gemtuzumab ozogamicin in acute myeloid leukemia: progress in understanding cytotoxicity and potential mechanisms of drug resistance.

CD33 is expressed on the malignant blast cells in most cases of acute myeloid leukemia (AML) but not on normal hematopoietic pluripotent stem cells. Antibody-based therapies for AML have, therefore, focused on CD33 as a suitable tumor-associated target antigen. The most promising results have been obtained with gemtuzumab ozogamicin (GO, Mylotarg), a humanized IgG(4) anti-CD33 monoclonal antibody joined to a calicheamicin-gamma(1) derivative. Engagement of CD33 by GO results in immunoconjugate internalization and hydrolytic release of the toxic calicheamicin moiety, which, in turn, causes DNA damage and cell death. Since 2000, when GO was approved for clinical use, treatment trials and pilot studies have revealed potential expanded applications along with additional limitations. At the same time, correlative biological and in vitro functional studies have further characterized CD33 expression patterns in AML, the significance of CD33-antibody interactions, pathways involved in GO-induced cytotoxicity and potential drug resistance mechanisms. This review summarizes the recent data addressing mechanisms of GO action and discusses their relevance with regard to clinical applications and the limitations of using experimental model systems to mimic in vivo conditions. As the first drug conjugate approved for clinical use, GO serves as an important paradigm for other immunoconjugates against internalizing tumor antigens.     

High CD33-antigen loads in peripheral blood limit the efficacy of gemtuzumab ozogamicin (Mylotarg) treatment in acute myeloid leukemia patients.

Gemtuzumab ozogamicin (Mylotarg) induces remission in approximately 30% of relapsed AML patients. We previously demonstrated that gemtuzumab infusion results in near-complete CD33 saturation in peripheral blood, and that saturating gemtuzumab levels result in continuous binding and internalization of gemtuzumab due to renewed CD33 expression. We now demonstrate that a high CD33-antigen load in peripheral blood is an independent adverse prognostic factor, likely due to peripheral consumption of gemtuzumab. Indeed, CD33 saturation in bone marrow is significantly reduced (40-90% saturation) as compared with CD33 saturation in corresponding peripheral blood samples (>90%). In vitro, such reduced CD33 saturation levels were strongly related with reduced cell kill. Apparently, high CD33-antigen loads in blood consume gemtuzumab and thereby limit its penetration into bone marrow. Consequently, CD33 saturation in bone marrow is reduced, which hampers efficient cell kill. Therefore, gemtuzumab should be administered at higher or repeated doses, or, preferably, after reduction of the leukemic cell burden by classical chemotherapy.     

The synergistic reversal effect of multidrug resistance by quercetin and hyperthermia in doxorubicin-resistant human myelogenous leukemia cells

Purpose: This study aimed to evaluate the multidrug resistance (MDR) reversal activity of quercetin (Que) in combination with hyperthermia (HT) in human myelogenous leukemia cells K562/A.

Methods: The cytotoxicity of Que alone and the effect of Que and HT to doxorubicin (Dox) cytotoxicity were determined using MTT assay in K562 and K562/A cells. K562/A cells was heated with or without Que pretreatment, and the protein and mRNA levels of heat shock protein 70 (HSP70) and P-glycoprotein (P-gp) were determined by flow cytometry (FCM) and RT-PCR, respectively. Intracellular accumulation of Dox, cell cycle and apoptosis were monitored with FCM.

Results: Que alone inhibited cell growth in a dose-dependent manner in K562 and K562/A cells. Either Que or HT alone had a weak reversal effect on Dox resistance, however, combination HT and Que showed a much more significant reversal effect on Dox resistance (reverse fold 9.49). The elevated protein expression and mRNA level of HSP70 and P-gp in response to HT were inhibited by Que. Pretreatment with Que caused the cells to accumulate Dox 8.3-fold higher than in control cells. In addition, Que induced apoptosis and G2/M arrest in a dose-dependent manner, and the combination of Que and HT was found to have a synergistic efeect on apoptosis.

Conclusions: Que pretreatment could significantly inhance the MDR reversal activity of HT in resistant cell line, by sensitizing the cell to reversing MDR activity of HT.     

Inhibition of P-glycoprotein by flavonoid derivatives in adriamycin-resistant human myelogenous leukemia (K562/ADM) cells.

We investigated the effects of natural flavones, quercetin and morin, and their pentamethyl, pentaethyl, pentapropyl, pentabutyl and pentaallyl ethers, on the function of P-glycoprotein (P-gp) assessed by an increase in the uptake of [3H]vincristine by human myelogenous leukemia (K562) cells and adriamycin-resistant human myelogenous leukemia (K562/ADM) cells. Pentamethyl, pentaethyl, pentapropyl and pentaallyl ethers of morin and quercetin (20 microM) all increased the uptake of [3H]vincristine by K562/ADM cells, while quercetin, morin and their pentabutyl ethers had no effect. Pentamethylquercetin, pentaallylquercetin and pentaethylmorin remarkably increased the uptake of [3H]vincristine by K562/ADM cells by 10.6, 10.8 and 14.4-fold, respectively. These inhibitory potencies for P-gp were more potent than typical P-gp inhibitors, cyclosporine A and verapamil. Taking into consideration that these flavonoid derivatives possess antitumor promoter activity, they may become candidates of effective multidrug resistance-reversing agents in cancer chemotherapy.     

The synergistic reversal effect of multidrug resistance by quercetin and hyperthermia in doxorubicin-resistant human myelogenous leukemia cells

PURPOSE: This study aimed to evaluate the multidrug resistance (MDR) reversal activity of quercetin (Que) in combination with hyperthermia (HT) in human myelogenous leukemia cells K562/A. METHODS: The cytotoxicity of Que alone and the effect of Que and HT to doxorubicin (Dox) cytotoxicity were determined using MTT assay in K562 and K562/A cells. K562/A cells was heated with or without Que pretreatment, and the protein and mRNA levels of heat shock protein 70 (HSP70) and P-glycoprotein (P-gp) were determined by flow cytometry (FCM) and RT-PCR, respectively. Intracellular accumulation of Dox, cell cycle and apoptosis were monitored with FCM. RESULTS: Que alone inhibited cell growth in a dose-dependent manner in K562 and K562/A cells. Either Que or HT alone had a weak reversal effect on Dox resistance, however, combination HT and Que showed a much more significant reversal effect on Dox resistance (reverse fold 9.49). The elevated protein expression and mRNA level of HSP70 and P-gp in response to HT were inhibited by Que. Pretreatment with Que caused the cells to accumulate Dox 8.3-fold higher than in control cells. In addition, Que induced apoptosis and G2/M arrest in a dose-dependent manner, and the combination of Que and HT was found to have a synergistic effect on apoptosis. CONCLUSIONS: Que pretreatment could significantly enhance the MDR reversal activity of HT in resistant cell line, by sensitizing the cell to reversing MDR activity of HT.     

Homing-associated cell adhesion molecule (H-CAM/CD44) on human CD34+ hematopoietic progenitor cells.

Human CD34+ hematopoietic progenitor cells (HPCs) express CD44 and can directly adhere to hyaluronate (HA) via CD44. Furthermore, CD44 may also be involved in the regulation of CD34+ HPC proliferation and development. The expression of CD44 molecules on CD34+ hematopoietic progenitor cells is significantly lower on bone marrow (BM) CD34+ cells compared with circulating CD34+ cells in cord blood and peripheral blood. Myeloid and erythroid progenitor cells are found predominantly in CD34+ CD44+ cell fractions. More interestingly, CD34+ CD44- cells expressing B-lymphocyte-associated CD10 and CD19 would represent unique B-lymphocyte committed precursors in the BM, which might undergo apoptotic cell death in the early steps of B-cell differentiation.     

beta(2)-microglobulin induces apoptosis in HL-60 human leukemia cell line and its multidrug resistant variants overexpressing MRP1 but lacking Bax or overexpressing P-glycoprotein.

In this study, we examined whether exogenous beta(2)-microglobulin (beta(2)m) can induce apoptosis in the drug sensitive HL-60 leukemia cell line and its drug resistant variants and investigated the molecular mechanism of beta(2)m-induced apoptosis. Our data revealed that beta(2)m is very significantly down-regulated in two multidrug resistant variants of the HL-60 cells: (a) the MRP1-bearing, Bax-deficient HL-60/ADR cell line, and (b) the P-glycoprotein (P-gp) overexpressing HL-60/VCR cell line. However, exogenous beta(2)m induced similar levels of apoptosis in HL-60 cells and these drug resistant variants. beta(2)m-induced apoptosis in HL-60 and HL-60/VCR cells was associated with decreased mitochondrial membrane potential (Deltapsim) but did not affect Deltapsim in HL-60/ADR cells. Surprisingly, cyclosporin A (CsA), a known inhibitor of the mitochondrial permeability transition (MPT) pore, inhibited beta(2)m-induced apoptosis in HL-60/ADR cells but not in HL-60 and HL-60/VCR cells, suggesting that the pro-apoptotic effect of beta(2)m in these cells is not through MPT pore formation. Furthermore, beta(2)m induced the release of cytochrome c and the apoptosis-inducing factor (AIF) from mitochondria in HL-60 and HL-60/VCR cells, but not in HL-60/ADR cells. Additionally, Z-VAD-fmk, a general inhibitor of caspases which inhibited cytochrome c release in HL-60 and HL-60/VCR cells, had no effect on AIF release in any of these cell lines, but inhibited beta(2)m-induced apoptosis in all three cell lines. However, Western blot analysis revealed that caspases-1, -3, -6, -8, and -9 are not activated during beta(2)m-induced apoptosis in these cells. Therefore, beta(2)m-induces apoptosis through an unknown caspase-dependent mitochondrial pathway in HL-60 and HL-60/VCR cells and by a Bax-independent, non-mitochondrial, caspase-dependent pathway in HL-60/ADR cells.     

VX-710 (biricodar) increases drug retention and enhances chemosensitivity in resistant cells overexpressing P-glycoprotein, multidrug resistance protein, and breast cancer resistance protein.

PURPOSE: The pipecolinate derivative VX-710 (biricodar; Incel) is a clinically applicable modulator of P-glycoprotein (Pgp) and multidrug resistance protein (MRP-1); we studied its activity against the third multidrug resistance (MDR)-associated drug efflux protein, breast cancer resistance protein (BCRP). EXPERIMENTAL DESIGN: VX-710 modulation of uptake, retention, and cytotoxicity of mitoxantrone, daunorubicin, doxorubicin, topotecan, and SN38 was studied in cell lines overexpressing Pgp, MRP-1 and wild-type (BCRP(R482)) and mutant (BCRP(R482T)) BCRP. RESULTS: In 8226/Dox6 cells (Pgp), VX-710 increased mitoxantrone and daunorubicin uptake by 55 and 100%, respectively, increased their retention by 100 and 60%, respectively, and increased their cytotoxicity 3.1- and 6.9-fold, respectively. In HL60/Adr cells (MRP-1), VX-710 increased mitoxantrone and daunorubicin uptake by 43 and 130%, increased their retention by 90 and 60%, and increased their cytotoxicity 2.4- and 3.3-fold. In 8226/MR20 cells (BCRP(R482)), VX-710 increased mitoxantrone uptake and retention by 60 and 40%, respectively, and increased cytotoxicity 2.4-fold. VX-710 increased daunorubicin uptake and retention by only 10% in 8226/MR20 cells, consistent with the fact that daunorubicin is not a substrate for BCRP(R482), but, nevertheless, it increased daunorubicin cytotoxicity 3.6-fold, and this increase was not associated with intracellular drug redistribution. VX-710 had little effect on uptake, retention, or cytotoxicity of mitoxantrone, daunorubicin, doxorubicin, topotecan, or SN38 in MCF7 AdVP3000 cells (BCRP(R482T)). CONCLUSIONS: VX-710 modulates Pgp, MRP-1, and BCRP(R482), and has potential as a clinical broad-spectrum MDR modulator in malignancies such as the acute leukemias in which these proteins are expressed.     

Augmentation by bispecific F(ab')2 reactive with P-glycoprotein and CD3 of cytotoxicity of human effector cells on P-glycoprotein positive human renal cancer cells.

A bispecific F(ba')2 was constructed that was composed of two Fab fragments, one derived from anti-CD3 monoclonal antibody (mAb) (OKT3) and the other from anti P-glycoprotein mAb (MRK 16). This bispecific F(ab')2 enhanced the binding and cytotoxicity of human peripheral blood mononuclear cells (PBMCs) on P-glycoprotein-positive human kidney cancer cells (ADMHK/E). It had no effect on the cytotoxicity of PBMCs on P-glycoprotein-negative HK/E cells [long-term cultured HK/E (LCHK/E)]. Control F(ab')2 composed of OKT3 or MRK16 alone did not influence the cytotoxicity of PBMCs on ADMHK/E cells. These findings suggest that the MRK16-OKT3 bispecific F(ab')2 may be therapeutically beneficial in treatment of human multidrug-resistant cancers.     

Coculture and transplant of purified CD34(+)Lin(-) and CD34(-)Lin(-) cells reveals functional interaction between repopulating hematopoietic stem cells.

The human hematopoietic stem cell compartment is comprised of repopulating CD34(+) and CD34(-) cells. The interaction between these subsets with respect to their reconstitution capacity in vivo remains to be characterized. Here, lineage-depleted (Lin(-)) human CD34(+) and CD34(-) hematopoietic cells were isolated from human male and female umbilical cord blood (CB) and transplanted into immune-deficient NOD/SCID EMV(null) mice, thereby allowing the use of human and Y-chromosome-specific DNA sequences to discriminate human reconstitution contributed by CD34(+) vs CD34(-) repopulating stem cells. Although cultured human CB CD34(-)Lin(-) cells transplanted alone possessed only minimal repopulating capacity, with 15% of mice achieving low levels of engraftment, transplantation of cocultured male CD34(-)Lin(-) cells with female CD34(+)Lin(-) cells demonstrated human repopulation with a contribution from CD34(-)Lin(-)-derived progeny in 80% of the recipients. After coculture and transplantation, male CD34(-)Lin(-) cells gave rise to primitive CD34(+)CD38(-) cells isolated in vivo, which demonstrated clonogenic progenitor function into multiple lineages. Taken together, our study indicates that the presence of CD34(+)Lin(-) cells in coculture enhanced the low repopulating function of human CD34(-)Lin(-) cells in vivo. We propose that CD34(+)Lin and CD34(-)Lin cells represent phenotypically distinct, but related cell types that exhibit unique and previously unappreciated functional interaction.     

c-myc protein kinetics during growth and differentiation of CD34 (MY10)-positive blast cells from normal human marrow

We have used flow cytometry to quantitate nuclear c-myc protein, at each phase of the cell cycle, during in-vitro differentiation of CD34-positive stem cells isolated from normal human bone marrow by the monoclonal antibody, MY10. Mean c-myc protein levels in CD34-positive cells, consisting of greater than 70% blasts, are lower than a marrow fraction containing myeloid cells of intermediate maturation, but have an invariant proportional relationship, with regard to nuclear mass, over the cell cycle. The majority of these primitive cells are non-cycling, as revealed by DNA content. Under our assay conditions, nuclear c-myc protein distribution over the cell cycle did not change as these progenitors entered a proliferative phase in culture. In cultures containing factors supporting myeloid maturation, mean G0/G1 p62c-myc levels initially decline, then rise above starting values as promyelocytes and myelocytes differentiate from CD34-positive cells, and as proliferation begins. With further myeloid maturation, and while cell numbers are increasing, c-myc protein continues to increase. C-myc protein kinetics differ in cultures in which macrophages, rather than myeloid cells, predominate. These data indicate that a complex relationship exists between c-myc gene expression and proliferation, maturation and lineage in haemopoietic cells, and lend support to the notion that early down regulation may be causally associated with the differentiation process.