Retroviral MDR1 gene transfer into marrow-engrafting human peripheral blood progenitor cells results in preferential transgene expression in the immature myeloid compartment rather than in mature myeloid progeny in vivo

BACKGROUND: The objective of multidrug resistance-1 (MDR1) gene therapy is protection of the myeloid cell lineage. It is therefore important to examine the effect of retroviral transduction on myeloid maturation. Transfer of the human MDR1 gene can confer resistance to a variety of cytostatic drugs. For a safe application in humans it is paramount to follow-up the development of transduced cells. METHODS: We transduced human mobilized peripheral blood progenitor cells (PBPC) with a viral vector containing the human MDR1 cDNA and transplanted the transduced cells into non-obese diabetic severe combined immunodeficient (NOD/SCID) mice. The progeny of the transduced cells was analyzed in detail by flow cytometry. RESULTS: A detailed analysis by four-color flow cytometry showed that MDR1 transgene-expressing CD33+ myeloid cells were preferentially negative for the maturation-associated myeloid markers CD11b and CD10, while the untransduced CD33+ myeloid cells expressed significantly higher proportions of these Ag (P<0.01 each). There was no difference in the expression of B- or T-lymphoid Ag among the MDR1-transduced and untransduced lymphoid cells. DISCUSSION: These data indicate that retroviral MDR1 gene transfer results in preferential P-glycoprotein expression in myeloid progenitor cells, which is the target cell population for myelotoxicity of cytostatic drugs.     

Antibody-targeted polymer-bound drugs

Drug targeting is an attractive new approach to killing cancer cells while leaving normal tissue unharmed. Recently we have developed a new generation of antibody-targeted immunosuppressive (cyclosporin A) and cytostatic (daunomycin, doxorubicin) drugs and photosensitizers (chlorin e₆) effectivein vitro andin vivo. The drugs and the targeting antibody (polyclonal and monoclonal) are conjugated to the oligopeptidic side chains of a water-soluble synthetic carrier, copolymer of N-(2-hydroxypropyl)methacrylamide. The composition of the side chains ensures the stability of the linkage between the drug and the polymeric carrier in the bloodstream and its intralysosomal degradability which is a prerequisite for the pharmacological activity of the preparation. Antibody-targeted polymer bound drugs show considerably decreased hepatotoxicity, cardiotoxicity, myelotoxicity and nephrotoxicity. Two adriamycin-HPMA copolymers are in Phase I/II clinical trials in United Kingdom.     

In vitro effects of human recombinant alpha-2b interferon on Ph1+ chronic myelogenous leukemia cells maintained in long term marrow cultures: a functional and morphological analysis

In this study in vitro results obtained with hu rec IFN-alpha 2b on Ph1+ stem cells from patients with chronic myelogenous leukemia in chronic phase (CML in CP) will be discussed: cells were incubated with different IFN concentrations (100, 1000, 10000 IU/ml) for different times (24, 96 hrs, 8, 15, days) and maintained in long term marrow cultures (LTMC); CFU-GM assay, cytochemistry and cytogenetic analyses were performed weekly. A high sensitivity of CML cells to the in vitro treatment with IFN was observed. Cell count in LTMC showed a progressive reduction inversely proportional to time of incubation and concentration of IFN; a marked decrease in colony growth was observed at the end of incubations and during the course of LTMC. Low concentrations of IFN permitted a morphological maturation and the expression of alkaline phosphatase. Cytogenetic analyses showed a marked reduction of mytoses in cultures treated with high concentrations of IFN as result of a combined cytostatic and cytolitic effect; the persistance of 100% Ph1+ cells in LTMC and in CFU-GM colonies might be related, as opposed to in vivo results, to different IFN exposure conditions or might be influenced by other factors.     

Sister chromatid exchange studies for monitoring DNA damage and repair capacity after cytostatics in vitro and in lymphocytes of leukaemic patients under cytostatic therapy.

The effect of various cytostatic drugs was studied on the frequency of sister chromatid exchanges (SCEs) in vitro and in PHA-stimulated lymphocytes of leukaemic patients under cytostatic therapy. The lymphocyte system is a sensitive one for the detection of DNA damage after administration of cytostatic drugs in vitro. Mitomycin C, busulphan, vincristine, chlorambucil, cytosine arabinoside, cyclophosphamide and lycurim were tested. All except cyclophosphamide induced high frequencies of SCEs in the first mitosis after their administration. The experiments with PHA-stimulated lymphocytes in vivo from patients treated with cytostatics showed that cytosine arabinoside, in combination with thioguanine, did not induce higher frequencies of SCEs, whereas in patients who were treated with cyclophosphamide alone or in combination with other cytostatic drugs, there was a higher incidence of SCEs during treatment. About 10 days after the termination of the treatment the elevated freuqencies of SCEs returned to the initial level. After administration of some mutagens, especially alkylating agents in vivo, the lymphocyte system can be used to assess induced DNA repair by continuously monitoring for SCEs.     

Differential toxicity of anticancer drugs on late (GM-CFC) and early (LTC-IC) hemopoietic progenitors in vitro

The clinical hematological toxicity of cytotoxic drugs can be acute, with a nadir of neutrophil count after 2 weeks and recovery the following week, or subacute, with a nadir of neutrophil count after 3 weeks and recovery in the following 2–3 weeks. The explanation usually given for this difference is that drugs in the first group are more toxic to mature hemopoietic precursors, while drugs of the second type are more toxic to undifferentiated cells. In an attempt to verify this hypothesis, we compared in vitro the effect of toxic doses of etoposide and tallimustine as representatives of drugs with acute toxicity, and of BCNU, melphalan, and carzelesin as representatives of drugs with subacute toxicity. Their effects were studied separately on more differentiated and earlier progenitors represented by granulocyte–macrophage colony-forming cells (GM-CFC) and long-term culture-initiating cells (LTC-IC), respectively. Etoposide, melphalan, BCNU, and carzelesin showed higher toxicity in differentiated than in early precursors: the concentration of drug inhibiting 70% (ID₇₀) of GM-CFC inhibited only by 10–40% the growth of LTC-IC. Tallimustine, in contrast, inhibited both GM-CFC and LTC-IC at comparable levels. These results do not correspond to the clinical pattern of myelotoxicity observed for those drugs. We conclude that the differential effects of antitumor drugs on later (GM-CFC) or earlier (LTC-IC) hemopoietic precursors may not represent a valid model for the pattern of myelotoxicity observed in humans. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular dynamics simulation study of tubulin dimer interaction with cytostatics

Colchicine, podophylotoxin, and indibulin are natural cytostatics that are used in the treatment of neoplasms. However, application of the compounds is restricted due to their high toxicity and low specificity. Computational experiments modeling tubulin interactions with the cytostatics seem a promising approach to design new analogues of the above-mentioned drugs with higher cytostatic activity and lower toxicity. Therefore, the CHARMM software was used to examine the macromolecules using molecular dynamics and mechanics methods. Particularly, a procedure was applied according to which molecules of each studied cytostatics were placed at several various random positions around the predicted binding site on tubulin. As a result, cytostatic binding regions were identified on the tubulin molecule. It was shown that, during the interaction, structural alterations occurred in these regions that may be responsible for tubulin polymerization. Thus, alterations have been revealed for the first time in the structure of tubulin in the regions of cytostatic binding that can substantially affect its function.     

An experimental model comparing the antineoplastic and the immunosuppressive effects of some cytostatics

An experimental model evaluating comparatively the antineoplastic and the immunosuppressive effects of some cytostatics (cyclophosphamide and vinblastine) is described in the present paper. Different cytostatic doses were intraperitoneally administered in mice which were 24 hours later grafted with a suspension of human tumoral KB cells. The xenograft acceptance was macroscopically and microscopically recorded 21 days later. By the Reed and Muench method and also by graphical extrapolation the LD50 (lethal dose for 50% animals) and the TD50 (the immunosuppressive dose enabling 50% animals to accept the xenografts) could thus be determined for both cyclophosphamide and vinblastine. The number (percent) of the tumour bearing animals three weeks after grafting was considered as an indicator of the cytostatic dose at which the immunosuppressive effect exceeded the antineoplastic effect. The in vitro effect of the same drugs on the KB cells was tested by inoculating different cytostatic doses in the cell cultures and counting at different time intervals the adherent as well as the nonadherent cells. The in vitro drug toxicity on the KB cell cultures was also determined by the trypan blue exclusion test. Both cyclophosphamide and vinblastine proved to be in vitro highly potent cytostatics i.e. when directly acting on the KB cells. This effect was dose correlated for both the considered drugs. However our in vivo experiments have shown that none of the observed effects when considering the direct action of the cytostatic on the cultured cells could not safely be extrapolated in vivo. Our results have an obvious practical importance when considering the therapeutical approaches in the neoplastic diseases. They demonstrate that the increase in cytostatic dose is not directly correlated to the antineoplastic effect since it reaches a limit at which the immunosuppressive effect highly exceed the tumour growth inhibition effect. The described experimental model could also be used in comparative estimations of the biological effects of different cytostatic drugs possibly referred to a standard immunosuppressive reagent as an antilymphocyte antiserum.     

Correlation of histopathological and biochemical appraisal of anthracyclin-induced myocardium damage

Treatment of various tumor types with anthracycl in cytostatic drugs (DNR--daunorubicin and DOX--doxorubicin) is limited by their high cardiotoxicity. This study was aimed at examining effectiveness of histopathological appraisal of myocardial cell injury induced by the cytostatic drugs and evaluated according to Billingham and by MTS methods as compared to results of parallel biochemical assessment of lipid peroxidation indices (MDA- malonyldialdehyde, 4-HDA-4-hydroxyalkenes). The experiments were performed on rats intoxicated with DNR or DOX in an acute manner (1 x 10 mg/kg body weight, i.v.) or a subchronic manner (3 x 3 mg/kg body weight. i.v.). Significant positive correlations were demonstrated between results of histological and biochemical appraisal in rats intoxicated in the acute manner with DNR (r=(0.91), intoxicated in the subchronic manner with DNR (r=0.90) and in rats intoxicated in the acute or subchronic manner with DOX (r=0.91, r=0.77, respectively). The obtained results have confirmed the free radical mechanism of cardiomyocyte injury induced by anthracyclines and the applied techniques of evaluating the destruction may be used independently of each other.     

Activation of Xenopus Eggs by the Kinase Inhibitor 6-DMAP Suggests a Differential Regulation of Cyclin B and p39 Proteolysis

In Xenopus eggs, metaphase II arrest is due to the cytostatic factor that maintains a high level of MPF activity. Kinases are important in this phenomenon since p39^mos and MAPK play a part in the cytostatic activity whereas p34^cdc2 is the catalytic subunit of MPF. Fertilization induces a rise in intracellular calcium leading to egg activation that can be mimicked by calcium-increasing agents such as calcium ionophore. We have performed on Xenopus eggs a biochemical comparison of the effects of the kinase inhibitor 6-DMAP and the calcium ionophore. Both drugs were able to induce pronucleus formation but the underlying molecular events were different. The inactivation of MAPK occurred earlier in eggs exposed to 6-DMAP. Cyclins B1 and B2 were stable and p39^mos was proteolysed in 6-DMAP-treated eggs while the three proteins underwent degradation in A23187-treated ones. These results suggest a differential regulation of ubiquitin-dependent proteolysis of cyclin B and p39^mos.     

The genotoxic risk of hospital, pharmacy and medical personnel occupationally exposed to cytostatic drugs--evaluation by the micronucleus assay

The aim of this study was to evaluate the genotoxicity of cytostatic drugs in hospital and pharmacy employees (n=100), occupationally exposed. The micronucleus assay was used to study lymphocytes in 247 peripheral blood samples. Samples were collected at "baseline level" without any cytostatic drugs exposure before recruiting or after at least 3 weeks without cytostatic drugs contact and at three times (cycle 1-3) post-exposure. Samples from 60 office employees served as controls. Furthermore, our results were compared to urinary analyses of cytostatic drugs (oxazaphosporines, anthracyclines, platinum) which were collected in parallel to the cytogenetic investigation. Statistical analyses were performed under consideration of age, gender and X-ray exposure. The frequency of micronuclei was significantly related to the age of the subjects (r(Spearman)=0.16; P<0.05). However, there were no significant differences in micronucleus rates between controls and exposed hospital workers. Similarly, micronucleus rates were not significantly different at the various sampling time points and there was no correlation between duration of employment and micronucleus rates. Furthermore, no correlation between current biomonitoring data of exposure (urine tests) and micronuclei frequency was found. Therefore, significantly increased genotoxic damage of the lymphocytes investigated in this study could not be demonstrated.     

Cells with marrow and spleen repopulating ability and forming spleen colonies on day 16, 12, and 8 are sequentially ordered on the basis of increasing rhodamine 123 retention

Mouse bone marrow cells (BMC) were subjected to countercurrent centrifugal elutriation and subsequently separated on the basis of light scatter and fluorescence intensity after being labeled with the supravital dye Rhodamine 123 (Rh-123). The sorted cells were then assayed for their in vivo spleen colony-forming ability (day -8, -12, and -16 CFU-S) and their ability to repopulate the bone marrow or spleen over a 13-day period with CFU-S-12, CFU-GM, or nucleated cells. Cells with marrow repopulating ability (MRA), as measured by the ability of the sorted cells to repopulate the marrow with secondary CFU-S-12 or CFU-GM, had low affinity for Rh-123. These cells showed minimal spleen colony-forming ability, and the ratio of MRA to CFU-S-12 in this preparation was 309. Cells with spleen repopulating ability (SRA), CFU-S-16, CFU-S-12, and CFU-S-8 retained increasing amounts of Rh-123, respectively, and CFU-S-8 were almost exclusively found among cells with high Rh-123 affinity. These cells also included about half of all day-12 CFU-S, and the ratio of MRA to day-12 CFU-S was 0. The results show that MRA cells, SRA cells, CFU-S-16, CFU-S-12, and CFU-S-8 can be sequentially ordered on the basis of increasing mitochondrial activity. The data also demonstrate for the first time, and without the application of negative selection by the use of cytostatic agents, that MRA cells are a separate class of primitive hemopoietic stem cells that fully meet the criteria of pre-CFU-S.     

Cell-mediated immunity is enhanced by cytostatic drugs continuously released at the site of antigenic stimulation

Summary Immunopotentiation by cytostatic drugs continuously released from osmotic minipumps, was investigated in a guinea-pig contact-sensitivity model. These pumps are designed to release their content within a period of 7 days. Vepesid (VP-16) and 5-fluorouracil were released into oxazolone-stimulated lymph nodes by subcutaneous implantation of pumps containing either of these drugs. The pumps were implanted at the intended sensitization site, 2 days before sensitization. Strong potentiation of T-cell-mediated immunity, evaluated by delayed-type hypersensitivity measurements, was observed with both drugs tested. Daily injections with VP-16 also caused an enhancement of the immune response. However, daily injections with 5-fluorouracil, a drug assumed to be cell-cycle-specific in its action, failed to potentiate delayed hypersensitivity to oxazolone. Intralesional administration of cytostatic drugs has been put forward as an effective treatment modality in various types of cancer. Therapeutic effects may depend on both local tumorcytotoxic and immunopotentiating activities. Our present results suggest that osmotic minipumps can be applied to broaden the applicability and effectiveness of local chemotherapy.     

Influence of human tumor suppressor PTEN on sensitivity of malignant cells to anticancer drugs

The influence of the human tumor suppressor PTEN on sensitivity of tumor cells to cytostatic drugs was studied. Rat ras-transformed (N-ras ^Asp12 ) fibroblasts were stably transfected with a full-size PTEN gene. Transfected clone was characterized by an enhanced expression of PTEN and a more normal phenotype in comparison with the parental cells. The effect of transient transfection with PTEN on the sensitivity of several malignant cell lines to the cytostatic drugs colchicine and adriablastine was studied. These drugs differ from each other in action mechanisms and intracellular targets. The tumor cell lines tested in this study included parental cell lines and stable sublines possessing drug resistance due to overexpression of P-glycoprotein. In all cell lines, introduction of exogenous PTEN caused a decrease in proliferation rates. This indicated that transgene was active. The chemosensitivity of some drug-resistant sublines was changed after PTEN transfection, but the drug sensitivity of parental cell lines remained unaffected. The effect of PTEN overexpression on chemosensitivity of malignant cells to cytostatic drugs was found to depend both on their mechanisms of action and on the origin of transfected cells. Our data suggest that PTEN is involved into the molecular mechanisms of drug resistance in cells studied.     

Vascular endothelium: target or victim of cytostatic therapy?

Chemotherapy continues to be the main therapeutic approach in the treatment of hematological malignancies including acute leukemia. Generally, chemotherapy is used to eliminate cancer cells and to restore normal bone marrow function. Simultaneous action of cytostatic drugs on bone marrow angiogenesis decreases the formation of new capillaries and improves therapeutic effect. However, chemotherapeutic agents may also be cytodestructive for cellular elements of other tissues, particularly the vascular endothelium, which can lead to various cardiovascular complications. In this work, we studied the effects of 2 cytostatic drugs, cytosine arabinoside (ara-C) and daunorubicin (DNR), on cultured human vascular (i.e., umbilical) endothelial cells (ECs). Ara-C and DNR were added to cultured cells at concentrations ranging from 1 ng/mL to 100 microg/mL. Drug effects were studied using phase-contrast microscopy, cell viability tests, BRDU incorporation, immunohistochemistry, flow cytometry, and cell cloning. At various concentrations, ara-C and DNR are able to induce morphological and functional changes in cultured cells related to either cytostatic or cytotoxic action. Moreover, ara-C-treated cultured cells displayed significant disturbances in cell adhesion molecule expression and interaction with blood leukocytes. Preliminary data obtained on acute leukemia patients undergoing standard cytostatic therapy ("7+3" regimen) have shown that concentration of the circulating ECs was significantly increased compared with the control group and could be as high as 500-1500 cells/mL of blood. Results obtained suggest that anticancer chemotherapy may induce systemic damage of vascular endothelium related to massive cell loss and (or) alterations of endothelial function.     

Calcium electroporation in three cell lines: a comparison of bleomycin and calcium,calcium compounds,and pulsing conditions

Background

Electroporation with calcium (calcium electroporation) can induce ATP depletion-associated cellular death. In the clinical setting, the cytotoxic drug bleomycin is currently used with electroporation (electrochemotherapy) for palliative treatment of tumors. Calcium electroporation offers several advantages over standard treatment options: calcium is inexpensive and may readily be applied without special precautions, as is the case with cytostatic drugs. Therefore, details on the use of calcium electroporation are essential for carrying out clinical trials comparing calcium electroporation and electrochemotherapy.

Methods

The effects of calcium electroporation and bleomycin electroporation (alone or in combination) were compared in three different cell lines (DC-3F, transformed Chinese hamster lung fibroblast; K-562, human leukemia; and murine Lewis Lung Carcinoma). Furthermore, the effects of electrical pulsing parameters and calcium compound on treatment efficacy were determined.

Results

Electroporation with either calcium or bleomycin significantly reduced cell survival (p < 0.0001), without evidence of a synergistic effect. Cellular death following calcium or bleomycin treatment occurred at similar applied voltages, suggesting that similar parameters should be applied. At equimolar concentrations, calcium chloride and calcium glubionate resulted in comparable decreases in cell viability.

Conclusions

Calcium electroporation and bleomycin electroporation significantly reduce cell survival at similar applied voltage parameters. The effect of calcium electroporation is independent of calcium compound.

General significance

This study strongly supports the use of calcium electroporation as a potential cancer therapy and the results may aid in future clinical trials.     

Inhibition of DNA synthesis is a potent mechanism by which cytostatic drugs induce homologous recombination in mammalian cells

Recombination is a process thought to be underlying genomic instability involved in carcinogenesis. This report examines the potential of cytostatic drugs to induce intrachromosomal homologous recombination. In order to address this question, the hprt gene of a well-characterized mammalian cell line was employed as a unique endogenous marker for homologous recombination. Commonly used cytostatic drugs with different mode of action were investigated in this context, i.e. bifunctional alkylating agents, inhibitors of DNA synthesis, inhibitors of topoisomerases and a spindle poison. With the exception of the spindle poison, all these drugs were found to induce homologous recombination, with clear differences in their recombination potency, which could be related to their mechanism of action. Bifunctional alkylating agents were the least efficient, whereas inhibitors of DNA synthesis were found to be the most potent inducers of homologous recombination. This raises the question whether these later drugs should be considered for adverse effects in cancer chemotheraphy.     

Selective killing of carcinoembryonic-antigen (CEA)-producing cells in vitro by the immunoconjugate cytorhodin-S and CEA-reactive cytorhodin-S antibody CA208

Summary Cytorhodin-S, an anthracycline derivative, was covalently coupled to a monoclonal antibody (mAb) CA208, against carcinoembryonic antigen (CEA) in order to achieve selective killing of a CEA-producing colon carcinoma cell line, COLO 205. The conjugate (15 molecules of drugs/antibody) retained substantial antibody activity as well as drug activity as assessed by enzyme-linked immunosorbent assay and 24-h L1210 proliferation assay, respectively. Furthermore, the conjugate inhibited the growth of COLO 205 cells in a short-term cytostatic assay. This cytostatic effect of the immunoconjugate on COLO 205 cells was inhibited in a dose-dependent manner by pretreatment of the cells with unconjugated CA208 mAb. In addition, chloroquine, a lysosomotropic agent, inhibited the cytostatic effect of the immunoconjugate, indicating the involvement of lysosomal enzymes in releasing drugs from the immunoconjugate. The antibody (CA208) was significantly incorporated into the cytoplasm of COLO 205 cells as demonstrated by immuno-electron microscopy. These in vitro results indicate that cytorhodin-S may be a good partner in immunoconjugates. However, in vivo animal experiments with the immunoconjugate revealed that the immunoconjugate was not so effective in prolonging survival. Thus, in vivo efficacy of this immunoconjugate remains to be further improved in application to cancer immunotherapy.     

Pulsed electric fields with calcium ions stimulate oxidative alternations and lipid peroxidation in human non-small cell lung cancer

Pulsed electric fields (PEFs) are commonly used to facilitate the delivery of various molecules, including pharmaceuticals, into living cells. However, the applied protocols still require optimization regarding the conditions of the permeabilization process, i.e., pulse waveform, voltage, duration, and the number of pulses in a burst. This study highlights the importance of electrochemical processes involved in the electropermeabilization process, known as electroporation. This research investigated the effects of electroporation on human non-small cell lung cancer cells (A549) in potassium (SKM) and HEPES-based buffers (SHM) using sub-microsecond and microsecond range pulses. The experiments were performed using 100 ns – 100 μs (0.6–15 kV/cm) bursts with 8 pulses in a sequence. It was shown that depending on the buffer composition, the susceptibility of cells to PEF varies, while calcium enhances the cytotoxic effects of PEF, if high cell membrane permeabilization is triggered. It was also determined that electroporation with calcium ions induces oxidative stress in cells, including lipid peroxidation (LPO), generation of reactive oxygen species (ROS), and neutral lipid droplets. Here, we demonstrated that calcium ions and optimized pulse parameters could potentiate PEF efficacy and oxidative alternations in lung cancer cells. Thus, the anticancer efficacy of PEF in lung cancers in combination with standard cytostatic drugs or calcium ions should be considered, but this issue still requires in-depth detailed studies with in vivo models.     

Expression of cell kinetics and death during monocyte–macrophage differentiation: effects of Actinomycin D and Vinblastine treatments

The different effects of two cytostatic drugs, Actinomycin D and Vinblastine, during macrophage-like differentiation induced in THP-1 monocytic cell line by phorbol ester phorbol 12-myristate 13-acetate (PMA) (6, 30, and 60 nM), were studied by morpho-cytochemical approaches. In PMA-unstimulated monocytic cells, the cytostatic effects of Actinomycin D (an antimetabolic drug) were characterized by a drastic reduction of the G2/M cells accompanied by dramatic death of the G1 cells; on the contrary, Vinblastine (a microtubule-depolymerizating drug) induced an accumulation of the G2/M cells with the appearance of aneugenic micronuclei and scarce cell death mainly from the G1 cells. After 60 nM PMA stimulation, the culture was mostly composed by macrophagic cells characterized by low proliferation and the appearance of mono-/binucleated polyploid cells; in this condition, the cytotoxicity of the two drugs, more effective for Vinblastine, induced cell death in the different ploidy classes (2c, 4c, 8c). Cell death appeared to be of apoptotic nature, but with some morpho-phenotypic differences due to the action mechanism of the drugs and dependent on cell culture growth and differentiation. As a consequence of the different block-action of the two drugs on the cell cycle phases and in relation to the different subcellular targets, the effects changed during the transition from not-adhering/proliferating monocytes to adhering/low-proliferating differentiated macrophages.     

Disturbances of desmofibrinogenesis in pateints suffering from acute myeloblastic leukemia

Significantly decreased levels of blood plasma clotting factor XIII (FSF) were found in the blood of 20 patients with acute myeloblastic leukemia as compared to the control values. It was found that after administration of cytostatic drugs (Cerubidyne and Cytosar) FSF deficiency was higher. This effect was associated with a proteolytic activity detectable in plasma which destroys FSF in vitro. This proteolytic activity was neither inhibited by EACA nor by Trasylol. These results indicate that in patients with acute myeloblastic leukemia beside DIC the treatment with cytostatic drugs as well as the presence of proteases from leukemic cells in the plasma will cause an impairment of transformation of soluble fibrin polymer into insoluble desmofibrin.