T Repka

Publications (10)74.44 Total impact

• Article: Fludarabine vs cladribine plus busulfan and low-dose TBI as reduced intensity conditioning for allogeneic hematopoietic stem cell transplantation: a prospective randomized trial.

  M Markova, J N Barker, J S Miller, M Arora, J E Wagner, L J Burns, M L MacMillan, D Douek, T DeFor, Y Tan, T Repka, B R Blazar, D J Weisdorf
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  ABSTRACT: Purine analogs are often used for conditioning preceding allogeneic hematopoietic stem cell transplantation (HCT). We prospectively tested fludarabine (Flu) 40 mg/m(2)/day x 5 days vs cladribine (Clad) 10 mg/m(2)/day x 5 days plus oral busulfan (1 mg/kg q6 h x 2 days) and total body irradiation 200 cGy in 32 recipients of matched sibling and unrelated donor (URD) HCT. Patients were similar in age (median 52 years), diagnosis, extensive pre-HCT therapy (56 vs 63%), and high-risk disease status (81 vs 93%). Neutrophil engraftment was prompt (median 11 vs 12 days), but early graft failure using Clad halted randomization. Platelet recovery was prompt (median Flu 18 vs Clad 24 days). Graft-versus-host disease (GVHD) after Flu vs Clad was similar; (acute grade II/IV 56 vs 69%, P=0.26; chronic 50 vs 31%, P=0.27). Nonrelapse mortality (Flu 25 vs Clad 38%, P=0.47) and progression-free survival at 3 years were similar as well. Multivariate analyses showed slightly, but not significantly lower relative risk (RR) of neutrophil engraftment with Clad (RR 0.6 (95% CI 0.2-1.3) P=0.16) and with URD RR 0.4 (0.2-1.0) P=0.04). Older patients with advanced hematologic malignancies achieve satisfactory outcomes using either of these reduced intensity conditioning regimens.
  Bone Marrow Transplantation 03/2007; 39(4):193-9. · 3.75 Impact Factor
• Article: Nonmyeloablative HPC transplantation.

  T Repka, D J Weisdorf
  Transfusion 08/2000; 40(7):758-60. · 3.22 Impact Factor
• Article: Natural killer cell cytotoxicity of breast cancer targets is enhanced by two distinct mechanisms of antibody-dependent cellular cytotoxicity against LFA-3 and HER2/neu.

  S Cooley, L J Burns, T Repka, J S Miller
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  ABSTRACT: Treatment of advanced breast cancer with autologous stem cell transplantation is limited by a high probability of disease relapse. In clinical trials, interleukin 2 (IL-2) alone can expand natural killer (NK) cells in vivo and increase their cytotoxic activity against breast cancer cell lines, but this increase is modest. Understanding the mechanisms that mediate NK cell lysis of breast cancer targets may lead to improvements of current immunotherapy strategies. NK cells from normal donors or patients receiving subcutaneous IL-2 were tested in cytotoxicity assays against five breast cancer cell lines. The role of adhesion molecules and antibodies that interact through Fc receptors on NK cells was explored. NK cell lysis of breast cancer targets is variable and is partially dependent on recognition through ICAM-1 and CD18. While blocking CD2 slightly decreased cytotoxicity, contrary to expectations, an antibody against CD58 (the ligand for CD2), failed to block killing and instead mediated an increased cytotoxicity that correlated with target density of CD58. The CD58 antibody-enhanced killing was dependent not only on FcRgammaIII but also on CD2 and ICAM-1/CD18. To further elucidate the mechanism of this CD58 antibody-dependent cellular cytotoxicity (ADCC), another antibody was tested. Trastuzumab (Herceptin), a humanized antibody against HER2/neu, mediated potent ADCC against all the HER2/neu positive breast cancer targets. Unlike CD58 antibody-mediated ADCC, Herceptin ADCC was minimally affected by blocking antibodies to CD2 or ICAM-1/CD18, which suggests a different mechanism of action. This study shows that multiple mechanisms are involved in NK cell lysis of breast cancer targets, that none of the targets are inherently resistant to killing, and that two distinct mechanisms of ADCC can target immunotherapy to breast cancer cells.
  Experimental Hematology 11/1999; 27(10):1533-41. · 2.90 Impact Factor
• Article: Peripheral blood versus bone marrow for hematopoietic cell transplantation.

  T Repka, D Weisdorf
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  ABSTRACT: Peripheral blood stem cell transplantation is being used more frequently in both the autologous and allogeneic setting. The use of cytokines either alone or in conjunction with chemotherapy priming has made peripheral procurement of the stem cells through apheresis possible. Differences exist both in the composition of the graft and in subsequent posttransplantation immune reconstitution between peripheral blood stem cells and bone marrow. Reliable estimates of the comparative incidence of acute and chronic graft-versus-host disease following blood or marrow allogeneic transplantation are not yet available.
  Current Opinion in Oncology 04/1998; 10(2):112-7. · 4.10 Impact Factor
• Source

  Available from: hematologylibrary.org

  Article: Deferiprone (L1) chelates pathologic iron deposits from membranes of intact thalassemic and sickle red blood cells both in vitro and in vivo.

  O Shalev, T Repka, A Goldfarb, L Grinberg, A Abrahamov, N F Olivieri, E A Rachmilewitz, R P Hebbel
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  ABSTRACT: Red blood cell (RBC) membranes from patients with the thalassemic and sickle hemoglobinopathies carry abnormal deposits of iron presumed to mediate a variety of oxidative-induced membrane dysfunctions. We hypothesized that the oral iron chelator deferiprone (L1), which has an enhanced capacity to permeate cell membranes, might be useful in chelating these pathologic iron deposits from intact RBCs. We tested this hypothesis in vitro by incubating L1 with RBCs from 15 patients with thalassemia intermedia and 6 patients with sickle cell anemia. We found that removal of RBC membrane free iron by L1 increased both as a function of time of incubation and L1 concentration. Thus, increasing the time of incubation of thalassemic RBCs with 0.5 mmol/L L1 from 0.5 to 6 hours, enhanced removal of their membrane free iron from 18% +/- 9% to 96% +/- 4%. Dose-response studies showed that incubating thalassemic RBC for 2 hours with L1 concentrations ranging from 0.125 to 0.5 mmol/L resulted in removal of membrane free iron from 28% +/- 15% to 68% +/- 11%. Parallel studies with sickle RBCs showed a similar pattern in time and dose responses. Deferoxamine (DFO), on the other hand, was ineffective in chelating membrane free iron from either thalassemic or sickle RBCs regardless of dose (maximum, 0.333 mmol/L) or time of incubation (maximum, 24 hours). In vivo efficacy of L1 was shown in six thalassemic patients whose RBC membrane free iron decreased by 50% +/- 29% following a 2-week course of L1 at a daily dose of 25 mg/kg. As the dose of L1 was increased to 50 mg/kg/d (n = 5), and then to 75 mg/kg/d (n = 4), 67% +/- 14% and 79% +/- 11%, respectively, of their RBC membrane free iron was removed. L1 therapy--both in vitro and in vivo--also significantly attenuated the malondialdehyde response of thalassemic RBC membranes to in vitro stimulation with peroxide. Remarkably, the heme content of RBC membranes from L1-treated thalassemic patients decreased by 28% +/- 10% during the 3-month study period. These results indicate that L1 can remove pathologic deposits of chelatable iron from thalassemic and sickle RBC membranes, a therapeutic potential not shared by DFO. Furthermore, membrane defects possibly mediated by catalytic iron, such as lipid peroxidation and hemichrome formation, may also be alleviated, at least in part, by L1.
  Blood 10/1995; 86(5):2008-13. · 9.90 Impact Factor
• Source

  Available from: hematologylibrary.org

  Article: Nonrandom association of free iron with membranes of sickle and beta-thalassemic erythrocytes.

  T Repka, O Shalev, R Reddy, J Yuan, A Abrahamov, E A Rachmilewitz, P S Low, R P Hebbel
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  ABSTRACT: To further define the nature of abnormal iron deposits on the membranes of pathologic red blood cells, we have used sickle cell anemia (HbSS), HbSC, and beta-thalassemic erythrocytes (RBCs) to prepare inside-out membranes (IOM) and insoluble membrane aggregates (AGGs) containing coclustered hemichrome and band 3. Study of IOM from HbSC and thalassemic patients showed that amounts of heme iron and, especially, free iron were much higher in patients who had undergone surgical splenectomy. The membrane AGGs from HbSS and beta-thalassemic RBCs contained much more globin than heme, with this discrepancy being variable from patient to patient. Although these AGGs were enriched (compared with the ghosts from which they were derived) for heme, as expected, less than 10% of total ghost heme was recovered in them. Remarkably, these AGGs also were enriched for nonheme iron, markedly so in some patients. Iron binding studies showed that the association of free iron with these hemichrome/band 3 AGGs is explained by the fact that free iron binds to denatured hemoglobin. These results document that free iron is nonrandomly associated with the membranes of sickle and beta-thalassemic RBCs. Whether this plays a causative role in the premature removal of such cells from the circulation remains to be seen.
  Blood 12/1993; 82(10):3204-10. · 9.90 Impact Factor
• Source

  Available from: nih.gov

  Article: Effect of excess alpha-hemoglobin chains on cellular and membrane oxidation in model beta-thalassemic erythrocytes.

  M D Scott, J J van den Berg, T Repka, P Rouyer-Fessard, R P Hebbel, Y Beuzard, B H Lubin
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  ABSTRACT: While red cells from individuals with beta thalassemias are characterized by evidence of elevated in vivo oxidation, it has not been possible to directly examine the relationship between excess alpha-hemoglobin chains and the observed oxidant damage. To investigate the oxidative effects of unpaired alpha-hemoglobin chains, purified alpha-hemoglobin chains were entrapped within normal erythrocytes. These "model" beta-thalassemic cells generated significantly (P < 0.001) greater amounts of methemoglobin and intracellular hydrogen peroxide than did control cells. This resulted in significant time-dependent decreases in the protein concentrations and reduced thiol content of spectrin and ankyrin. These abnormalities correlated with the rate of alpha-hemoglobin chain autoxidation and appearance of membrane-bound globin. In addition, alpha-hemoglobin chain loading resulted in a direct decrease (38.5%) in catalase activity. In the absence of exogenous oxidants, membrane peroxidation and vitamin E levels were unaltered. However, when challenged with an external oxidant, lipid peroxidation and vitamin E oxidation were significantly (P < 0.001) enhanced in the alpha-hemoglobin chain-loaded cells. Membrane bound heme and iron were also significantly elevated (P < 0.001) in the alpha-hemoglobin chain-loaded cells and lipid peroxidation could be partially inhibited by entrapment of an iron chelator. In contrast, chemical inhibition of cellular catalase activity enhanced the detrimental effects of entrapped alpha-hemoglobin chains. In summary, entrapment of purified alpha-hemoglobin chains within normal erythrocytes significantly enhanced cellular oxidant stress and resulted in pathological changes characteristic of thalassemic cells in vivo. This model provides a means by which the pathophysiological effects of excess alpha-hemoglobin chains can be examined.
  Journal of Clinical Investigation 04/1993; 91(4):1706-12. · 15.39 Impact Factor
• Source

  Available from: nih.gov

  Article: Detection, characterization, and bioavailability of membrane-associated iron in the intact sickle red cell.

  T Sugihara, T Repka, R P Hebbel
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  ABSTRACT: It is hypothesized that membrane-associated iron in the sickle red cell is of pathophysiologic importance, but the actual existence of such iron in the intact cell has been questioned. Using a strategy whereby membrane iron can be detected through its bioavailability for catalyzing peroxidation, we used phospholipid exchange protein to load membranes of intact erythrocytes (RBC) with approximately 2% phosphatidylethanolamine hydroperoxide (PEOOH) and monitored the development of peroxidation by-products during subsequent incubation. Normal RBC loaded with PEOOH developed very little peroxidation, but vitamin E-replete sickle RBC showed an exuberant peroxidation response that was not seen in cells loaded with control nonoxidized phosphatidylethanolamine. Ancillary studies of sickle RBC revealed that the catalytic iron included both heme iron and free iron located at the bilayer inner leaflet. Significantly, these studies also revealed that peroxidation after PEOOH loading is promoted by cellular dehydration and inhibited by hydration, thus identifying a dynamic interaction between hemoglobin (sickle > normal) and membrane lipid. High-reticulocyte control RBC and sickle trait RBC behaved exactly like normal RBC, while HbCC RBC and RBC having membranes gilded with hemoglobin iron because of prior exposure to acetylphenylhydrazine showed an abnormal peroxidation response like that of sickle RBC. Indeed, the peroxidation response of RBC loaded with PEOOH paralleled amounts of iron measured on inside-out membranes prepared from them (r = 0.783, P < 0.01). These studies corroborate existence of membrane-associated heme and free iron in the intact sickle cell, and they document its bioavailability for participation in injurious peroxidative processes. That association of cytosolic sickle hemoglobin with membrane lipid is modulated by cell hydration status provides a mechanism that may help explain increased development of oxidative membrane lesions in abnormally dehydrated sickle RBC regardless of the mechanism underlying their formation.
  Journal of Clinical Investigation 12/1992; 90(6):2327-32. · 15.39 Impact Factor
• Source

  Available from: hematologylibrary.org

  Article: Hydroxyl radical formation by sickle erythrocyte membranes: role of pathologic iron deposits and cytoplasmic reducing agents.

  T Repka, R P Hebbel
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  ABSTRACT: Sickle erythrocyte (RBC) membranes were previously shown to manifest increased Fenton activity (iron-dependent, peroxide-driven formation of hydroxyl radical [.OH]) compared with normal RBC membranes, but the nature of the catalytic iron was not defined. We now find that sickle membranes exposed to superoxide (.O2-) and hydrogen peroxide (H2O2) have three distinct iron compartments able to act as Fenton catalysts: preexisting free iron, free iron released during oxidant stress, and a component that cannot be chelated with deferoxamine (DF). In a model system, addition of iron compounds to normal ghosts showed that free heme, hemoglobin, Fe/adenosine diphosphate (ADP), and ferritin all catalyze .OH production; concurrent inhibition studies using DF documented that the unchelatable Fenton component is free heme or hemoglobin. During exposure to peroxide only, the iron in sickle membranes was unable to act as a Fenton catalyst without addition of a reducing agent. At physiologic concentrations, both ascorbate and glutathione restored Fenton activity. Lipid peroxidation studies showed that at physiologic levels ascorbate acts primarily as an antioxidant; however, as pharmacologic levels are reached, its pro-oxidant effects predominate. This study elucidates the catalytic ability of the iron compartments in the sickle cell membrane, the importance of which relates to the potential role of .OH in membrane damage. It also illustrates the potential participation of cytoplasmic reducing agents in this process, which may be especially relevant in the context of administration of supraphysiologic doses of ascorbate to sickle cell patients.
  Blood 12/1991; 78(10):2753-8. · 9.90 Impact Factor
• Article: Reticulocytosis: defective reticulin degradation, a case report.

  T Repka, B H Rank
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  ABSTRACT: In summary, this case illustrates a previously unrecognized syndrome of reticulocytosis secondary to a defect in reticulin degradation. We are aware of two other case reports in the literature which also describe patients with an elevated reticulocyte count who's red cells appeared to lack the ability to degrade reticulin (Lofters et. al.1978, Tulliez et. al. 1982). Both of these cases however, demonstrated only mild reticulocytosis and appeared to be related to preleukemic syndromes. Our patient had no evidence of preleukemia and indeed, has been followed for over four years without evidence of other hematologic abnormalities. The mild anemia that has persisted in this patient may suggest that although no evidence of hemolysis is obvious in this patient, some minor amount of hemolysis may be occurring. Due to the relative insensitivity of other measures of early red cell death (51Cr survival, bone marrow biopsy, etc...), and the loss of the the reticulocyte count as a sensitive index of hemolysis, a minor amount of hemolysis cannot be excluded. This does not decrease the validity of the observations noted above, since this massive reticulocytosis cannot be explained by minimal hemolysis. Whether this patient's reticulocytosis represents an acquired or congenital syndrome is unsettled at present. Although his brother is unaffected, and we have on record only a mildly elevated reticulocyte count from 1981, there remains a possibility that this syndrome is congenital. We are, at present, in the process of further characterizing the specific defect in these red cells. Further assays of Lipoxygenase and protease activity, the ability to ubiquitinate proteins, specific identification of the mitochondria-like structures, and protein synthesis assays are in progress or are planned. The inability to degrade reticulin in this patient's red cells and the impact of this upon other cellular physiologic processes afford an excellent opportunity to enhance our understanding of normal red cell physiology, particularly, we suspect in this patient, of Ubiquitin-ATP-dependent proteolysis.
  Progress in clinical and biological research 02/1989; 319:291-300; discussion 301-2.