Isolated AL Amyloidosis of the Colon: A Rare Presentation.

Amyloidosis is a group of rare deposition diseases marked by the accumulation of abnormal fibrillar proteins in the extracellular space of various tissues. In both AL and AA amyloidosis, the most common variants, isolated involvement to any one organ is uncommon and involvement to the colon alone is especially rare. We present the case of a patient who was initially found to have AL amyloidosis on prior screening colonoscopy that was reconfirmed several years with repeat evaluation for chronic constipation. This disease process is often insidious and can be overlooked by providers given the lack of overwhelming symptoms.

Multifocal Follicular Lymphoma Isolated to the Gastrointestinal Tract: A Rare Finding.

Follicular lymphoma (FL) is a common form of non-Hodgkin lymphoma. Although extranodal involvement of the gastrointestinal (GI) tract is common in lymphomas, primary GI-FL confined to the GI tract is relatively rare. The disease process is typically indolent in nature and usually incidentally found. Among this subset of patients, the duodenum and terminal ileum tend to be the most common site of origin. Here, we present a rare case of primary multifocal GI-FL that found incidentally during routine colonoscopy with subsequent esophagogastroduodenoscopy and video capsule endoscopy revealing duodenal, jejunal, and sigmoid colon involvement.

Past, Present, and a Glance into the Future of Multiple Myeloma Treatment.

Multiple myeloma (MM) is a challenging hematological cancer which typically grows in bone marrow. MM accounts for 10% of hematological malignancies and 1.8% of cancers. The recent treatment strategies have significantly improved progression-free survival for MM patients in the last decade; however, a relapse for most MM patients is inevitable. In this review we discuss current treatment, important pathways for proliferation, survival, immune suppression, and resistance that could be targeted for future treatments.

North American Blastic Plasmacytoid Dendritic Cell Neoplasm Consortium: position on standards of care and areas of need.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare hematologic malignancy with historically poor outcomes and no worldwide consensus treatment approach. Unique among most hematologic malignancies for its frequent cutaneous involvement, BPDCN can also invade other extramedullary compartments, including the central nervous system. Generally affecting older adults, many patients are unfit to receive intensive chemotherapy, and although hematopoietic stem cell transplantation is preferred for younger, fit individuals, not all are eligible. One recent therapeutic breakthrough is that all BPDCNs express CD123 (IL3Rα) and that this accessible surface marker can be pharmacologically targeted. The first-in-class agent for BPDCN, tagraxofusp, which targets CD123, was approved in December 2018 in the United States for patients with BPDCN aged ≥2 years. Despite favorable response rates in the frontline setting, many patients still relapse in the setting of monotherapy, and outcomes in patients with relapsed/refractory BPDCN remain dismal. Therefore, novel approaches targeting both CD123 and other targets are actively being investigated. To begin to formally address the state of the field, we formed a new collaborative initiative, the North American BPDCN Consortium (NABC). This group of experts, which includes a multidisciplinary panel of hematologists/oncologists, hematopoietic stem cell transplant physicians, pathologists, dermatologists, and pediatric oncologists, was tasked with defining the current standard of care in the field and identifying the most important research questions and future directions in BPDCN. The position findings of the NABC's inaugural meetings are presented herein.

Development of a novel Bruton's tyrosine kinase inhibitor that exerts anti-cancer activities potentiates response of chemotherapeutic agents in multiple myeloma stem cell-like cells.

Despite recent improvements in multiple myeloma (MM) treatment, MM remains an incurable disease and most patients experience a relapse. The major reason for myeloma recurrence is the persistent stem cell-like population. It has been demonstrated that overexpression of Bruton's tyrosine kinase (BTK) in MM stem cell-like cells is correlated with drug resistance and poor prognosis. We have developed a novel small BTK inhibitor, KS151, which is unique compared to other BTK inhibitors. Unlike ibrutinib, and the other BTK inhibitors such as acalabrutinib, orelabrutinib, and zanubrutinib that covalently bind to the C481 residue in the BTK kinase domain, KS151 can inhibit BTK activities without binding to C481. This feature of KS151 is important because C481 becomes mutated in many patients and causes drug resistance. We demonstrated that KS151 inhibits in vitro BTK kinase activities and is more potent than ibrutinib. Furthermore, by performing a semi-quantitative, sandwich-based array for 71-tyrosine kinase phosphorylation, we found that KS151 specifically inhibits BTK. Our western blotting data showed that KS151 inhibits BTK signaling pathways and is effective against bortezomib-resistant cells as well as MM stem cell-like cells. Moreover, KS151 potentiates the apoptotic response of bortezomib, lenalidomide, and panobinostat in both MM and stem cell-like cells. Interestingly, KS151 inhibits stemness markers and is efficient in inhibiting Nanog and Gli1 stemness markers even when MM cells were co-cultured with bone marrow stromal cells (BMSCs). Overall, our results show that we have developed a novel BTK inhibitor effective against the stem cell-like population, and potentiates the response of chemotherapeutic agents.

CD30+ Lymphoproliferative Disorders as Potential Candidates for CD30-Targeted Therapies.

CONTEXT.­: In the early 1980s, a monoclonal antibody termed Ki-1 was developed against a cell line derived from a patient with Hodgkin lymphoma. This antibody detected a limited number of benign activated lymphocytes in lymphoid tissue, whereas in Hodgkin lymphoma it appeared to be nearly specific for Reed-Sternberg cells and their mononuclear variants. Subsequent studies showed that Ki-1 expression defined a new type of lymphoma that was later designated anaplastic large cell lymphoma with or without anaplastic large cell kinase expression/translocation. In the past 30 years, numerous new lymphoma entities have been defined, many of which are variably positive for CD30. Many virally transformed lymphoproliferative disorders are also frequently positive for CD30. OBJECTIVE.­: To illustrate the broad spectrum of CD30+ hematologic malignancies and to provide an update of CD30-targeted therapies. DATA SOURCES.­: Personal experiences and published works in PubMed. CONCLUSIONS.­: Because of its low expression in normal tissue, CD30 was studied as a therapeutic target for many years. However, the first functional humanized antibody against CD30 was developed only about 10 years ago. Brentuximab vedotin is a humanized anti-CD30 antibody linked to a cytotoxin, and was approved by the US Food and Drug Administration in 2012 for treating refractory Hodgkin lymphoma and anaplastic large cell lymphoma. Since then, the list of Food and Drug Administration-approved CD30-targeted hematologic malignancies has grown. Recently, the therapies using tumor antigen-specific chimeric antigen receptor T cells targeting CD30 have incited a great deal of enthusiasm and are studied in clinical trials.

Autophagy and Apoptosis: Current Challenges of Treatment and Drug Resistance in Multiple Myeloma.

Over the past two decades, the natural history of multiple myeloma (MM) has evolved dramatically, owing primarily to novel agents targeting MM in the bone marrow microenvironment (BMM) pathways. However, the mechanisms of resistance acquisition remain a mystery and are poorly understood. Autophagy and apoptosis are tightly controlled processes and play a critical role in the cell growth, development, and survival of MM. Genetic instability and abnormalities are two hallmarks of MM. During MM progression, plasma malignant cells become genetically unstable and activate various signaling pathways, resulting in the overexpression of abnormal proteins that disrupt autophagy and apoptosis biological processes. Thus, achieving a better understanding of the autophagy and apoptosis processes and the proteins that crosslinked both pathways, could provide new insights for the MM treatment and improve the development of novel therapeutic strategies to overcome resistance. This review presents a sufficient overview of the roles of autophagy and apoptosis and how they crosslink and control MM progression and drug resistance. Potential combination targeting of both pathways for improving outcomes in MM patients also has been addressed.

Bruton's Tyrosine Kinase Targeting in Multiple Myeloma.

Multiple myeloma (MM), a clonal plasma cell disorder, disrupts the bones' hematopoiesis and microenvironment homeostasis and ability to mediate an immune response against malignant clones. Despite prominent survival improvement with newer treatment modalities since the 2000s, MM is still considered a non-curable disease. Patients experience disease recurrence episodes with clonal evolution, and with each relapse disease comes back with a more aggressive phenotype. Bruton's Tyrosine Kinase (BTK) has been a major target for B cell clonal disorders and its role in clonal plasma cell disorders is under active investigation. BTK is a cytosolic kinase which plays a major role in the immune system and its related malignancies. The BTK pathway has been shown to provide survival for malignant clone and multiple myeloma stem cells (MMSCs). BTK also regulates the malignant clones' interaction with the bone marrow microenvironment. Hence, BTK inhibition is a promising therapeutic strategy for MM patients. In this review, the role of BTK and its signal transduction pathways are outlined in the context of MM.

Promyelocytic Blast Crisis of Chronic Myeloid Leukemia in a Patient Undergoing Therapy with a Tyrosine Kinase Inhibitor.

A 58-year-old male with the chronic phase of chronic myeloid leukemia (CML), treated with a tyrosine kinase inhibitor (TKI), bosutinib, since the past two years, presented with bright red bleeding per rectum and disseminated intravascular coagulation. A bone marrow biopsy reverse transcription-polymerase chain reaction revealed a promyelocytic blast crisis, with leukemic cells displaying both BCR/ABL and PML/RARα chimeric genes. Cytogenetic studies revealed translocations of both t(15;17) and t(9;22). With the initiation of all-trans retinoic acid, arsenic trioxide and gemtuzumab, the patient achieved remission, with absent PML/RARα by fluorescence in situ hybridization analysis. This case highlights the importance of long-term monitoring of patients with CML, especially those on TKIs, for the development of secondary leukemias in the future.

The Effect of the Histone Deacetylase Inhibitor Suberoylanilide Hydroxamic Acid and Paclitaxel Treatment on Full-Thickness Wound Healing in Mice.

INTRODUCTION: Neoadjuvant chemotherapy prior to lumpectomy or mastectomy for breast cancer challenges wound healing. Suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, has been shown to work synergistically with paclitaxel in vitro and in preclinical studies. In addition, our laboratory has demonstrated that SAHA treatment decreases paclitaxel-associated stem cell toxicity, modulates inflammatory response, and promotes wound healing in injured fibroblast cells. Our goal was to determine if combined SAHA and paclitaxel treatment would improve wound healing in an in vivo full-thickness murine model, without altering antitumor effect. METHODS: Thirty-two nude athymic mice received intraperitoneal injections of paclitaxel (20 mg/kg), SAHA (25 mg/kg), paclitaxel + SAHA (20 mg/kg + 25 mg/kg), or no treatment for 2 weeks prior to surgery. Under general anesthesia, 8-mm full-thickness dorsal wounds were created in all animals, and a silicone splint was attached to minimize wound contraction. The wounds were measured twice a week with a surgical caliper until healing was complete. To evaluate the in vivo effect of drug treatment, 16 athymic nude mice with MDA-MB-231 xenografts received the treatments described previously, following which tumor volumes were compared between groups. RESULTS: Average wound healing time was prolonged in mice treated with paclitaxel (20 ± 1.9 days), and combination SAHA + paclitaxel therapy improved average wound healing time (17.0 ± 1.8 days). In the xenograft model, the antitumor effect of SAHA and paclitaxel (average tumor volume 43.9 ± 34.1 mm) was greater than paclitaxel alone (105.8 ± 73.8 mm). CONCLUSIONS: The addition of SAHA to taxane chemotherapy improves the therapeutic effect on triple-negative breast cancer while decreasing the detrimental effect of paclitaxel on wound healing. This may have substantial implications on improving outcomes in breast reconstruction following chemotherapy.

New interleukin-15 superagonist (IL-15SA) significantly enhances graft-versus-tumor activity.

Interleukin-15 (IL-15) is a potent cytokine that increases CD8+ T and NK cell numbers and function in experimental models. However, obstacles remain in using IL-15 therapeutically, specifically its low potency and short in vivo half-life. To help overcome this, a new IL-15 superagonist complex comprised of an IL-15N72D mutation and IL-15RαSu/Fc fusion (IL-15SA, also known as ALT-803) was developed. IL-15SA exhibits a significantly longer serum half-life and increased in vivo activity against various tumors. Herein, we evaluated the effects of IL-15SA in recipients of allogeneic hematopoietic stem cell transplantation. Weekly administration of IL-15SA to transplant recipients significantly increased the number of CD8+ T cells (specifically CD44+ memory/activated phenotype) and NK cells. Intracellular IFN-γ and TNF-α secretion by CD8+ T cells increased in the IL-15SA-treated group. IL-15SA also upregulated NKG2D expression on CD8+ T cells. Moreover, IL-15SA enhanced proliferation and cytokine secretion of adoptively transferred CFSE-labeled T cells in syngeneic and allogeneic models by specifically stimulating the slowly proliferative and nonproliferative cells into actively proliferating cells.We then evaluated IL-15SA's effects on anti-tumor activity against murine mastocytoma (P815) and murine B cell lymphoma (A20). IL-15SA enhanced graft-versus-tumor (GVT) activity in these tumors following T cell infusion. Interestingly, IL-15 SA administration provided GVT activity against A20 lymphoma cells in the murine donor leukocyte infusion (DLI) model without increasing graft versus host disease. In conclusion, IL-15SA could be a highly potent T- cell lymphoid growth factor and novel immunotherapeutic agent to complement stem cell transplantation and adoptive immunotherapy.

A novel dual inhibitor of microtubule and Bruton's tyrosine kinase inhibits survival of multiple myeloma and osteoclastogenesis.

Bruton's tyrosine kinase (BTK) regulates many vital signaling pathways and plays a critical role in cell proliferation, survival, migration, and resistance. Previously, we reported that a small molecule, KS99, is an inhibitor of tubulin polymerization. In the present study, we explored whether KS99 is a dual inhibitor of BTK and tubulin polymerization. Although it is known that BTK is required for clonogenic growth and resistance, and microtubules are essential for cancer cell growth, dual targeting of these two components has not been explored previously. Through docking studies, we predicted that KS99 interacts directly with the catalytic domain of BTK and inhibits phosphorylation at the Y223 residue and kinase activities. Treatment of KS99 reduces the cell viability of multiple myeloma (MM) and CD138+ cells, with an IC50 of between 0.5 and 1.0 µmol/L. We found that KS99 is able to induce apoptosis in MM cells in a caspase-dependent manner. KS99 suppressed the receptor activator of NF-κB ligand (RANKL)-induced differentiation of macrophages to osteoclasts in a dose-dependent manner and, importantly, inhibited the expression of cytokines associated with bone loss. Finally, we found that KS99 inhibits the in vivo tumor growth of MM cells through the inhibition of BTK and tubulin. Overall, our results show that dual inhibition of BTK and tubulin polymerization by KS99 is a viable option in MM treatment, particularly in the inhibition of refraction and relapse.

Cobblestone-area forming cells derived from patients with mantle cell lymphoma are enriched for CD133+ tumor-initiating cells.

Mantle cell lymphoma (MCL) is associated with a significant risk of therapeutic failure and disease relapse, but the biological origin of relapse is poorly understood. Here, we prospectively identify subpopulations of primary MCL cells with different biologic and immunophenotypic features. Using a simple culture system, we demonstrate that a subset of primary MCL cells co-cultured with either primary human mesenchymal stromal cells (hMSC) or murine MS-5 cells form in cobblestone-areas consisting of cells with a primitive immunophenotype (CD19-CD133+) containing the chromosomal translocation t (11;14)(q13;q32) characteristic of MCL. Limiting dilution serial transplantation experiments utilizing immunodeficient mice revealed that primary MCL engraftment was only observed when either unsorted or CD19-CD133+ cells were utilized. No engraftment was seen using the CD19+CD133- subpopulation. Our results establish that primary CD19-CD133+ MCL cells are a functionally distinct subpopulation of primary MCL cells enriched for MCL-initiating activity in immunodeficient mice. This rare subpopulation of MCL-initiating cells may play an important role in the pathogenesis of MCL.

Enrichment of human prostate cancer cells with tumor initiating properties in mouse and zebrafish xenografts by differential adhesion.

BACKGROUND: Prostate tumor-initiating cells (TICs) have intrinsic resistance to current therapies. TICs are commonly isolated by cell sorting or dye exclusion, however, isolating TICs from limited primary prostate cancer (PCa) tissues is inherently inefficient. We adapted the collagen adherence feature to develop a combined immunophenotypic and time-of-adherence assay to identify human prostate TICs. METHODS: PCa cells from multiple cell lines and primary tissues were allowed to adhere to several matrix molecules, and fractions of adherent cells were examined for their TIC properties. RESULTS: Collagen I rapidly-adherent PCa cells have significantly higher clonogenic, migration, and invasion abilities, and initiated more tumor xenografts in mice when compared to slowly-adherent and no-adherent cells. To determine the relative frequency of TICs among PCa cell lines and primary PCa cells, we utilized zebrafish xenografts to define the tumor initiation potential of serial dilutions of rapidly-adherent α2ß1(hi) /CD44(hi) cells compared to non-adherent cells with α2ß1(low) /CD44(low) phenotype. Tumor initiation from rapidly-adherent α2ß1(hi) /CD44(hi) TICs harboring the TMPRSS2:ERG fusion generated xenografts comprising of PCa cells expressing Erg, AMACR, and PSA. Moreover, PCa-cell dissemination was consistently observed in the immune-permissive zebrafish microenvironment from as-few-as 3 rapidly-adherent α2ß1(hi) /CD44(hi) cells. In zebrafish xenografts, self-renewing prostate TICs comprise 0.02-0.9% of PC3 cells, 0.3-1.3% of DU145 cells, and 0.22-14.3% of primary prostate adenocarcinomas. CONCLUSION: Zebrafish PCa xenografts were used to determine that the frequency of prostate TICs varies among PCa cell lines and primary PCa tissues. These data support a paradigm of utilizing zebrafish xenografts to evaluate novel therapies targeting TICs in prostate cancer.

P53 regulates the migration of mesenchymal stromal cells in response to the tumor microenvironment through both CXCL12-dependent and -independent mechanisms.

Mesenchymal stromal cells (MSCs) are multipotent fibroblast-like cells located in the bone marrow that localize to areas of tissue damage including wounds and solid tumors. Within the tumor microenvironment, MSCs adopt the phenotype of carcinoma-associated fibroblasts (CAFs) and stimulate tumor growth. Production of the chemokine CXCL12, also known as stromal cell-derived factor 1 (SDF-1), by MSCs is required for their in vitro migration in response to tumor cells and has also been implicated in stimulation of tumor growth. The tumor suppressor p53 regulates cellular migration, CXCL12 production and the promotion of tumor growth by carcinoma-associated fibroblasts (CAFs). We investigated the role of p53 in MSC migration to tumors. P53 inhibits the migration of MSCs in response to tumor cells in conjunction with a decrease in CXCL12 transcription. Conversely, decreased p53 activity leads to enhanced MSC migration. Interestingly, increased p53 activity inhibits MSC migration even in the context of high concentrations of exogenous CXCL12. These data show that stromal p53 status impacts the recruitment of MSCs to solid tumors through both regulation of CXCL12 production as well as other mechanisms. Stromal p53 may influence other important aspects of tumor biology such as tumor growth and metastasis through mechanisms distinct from CXCL12.

Antitumor and modeling studies of a penetratin-peptide that targets E2F-1 in small cell lung cancer.

E2F-1, a key transcription factor necessary for cell growth, DNA repair, and differentiation, is an attractive target for development of anticancer drugs in tumors that are E2F "oncogene addicted". We identified a peptide isolated from phage clones that bound tightly to the E2F-1 promoter consensus sequence. The peptide was coupled to penetratin to enhance cellular uptake. Modeling of the penetratin-peptide (PEP) binding to the DNA E2F-1 promoter demonstrated favorable interactions that also involved the participation of most of the penetratin sequence. The penetratin-peptide (PEP) demonstrated potent in vitro cytotoxic effects against a range of cancer cell lines, particularly against Burkitt lymphoma cells and small cell lung cancer (SCLC) cells. Further studies in the H-69 SCLC cell line showed that the PEP inhibited transcription of E2F-1 and also several important E2F-regulated enzymes involved in DNA synthesis, namely, thymidylate synthase, thymidine kinase, and ribonucleotide reductase. As the PEP was found to be relatively unstable in serum, it was encapsulated in PEGylated liposomes for in vivo studies. Treatment of mice bearing the human small cell lung carcinoma H-69 with the PEP encapsulated in PEGylated liposomes (PL-PEP) caused tumor regression without significant toxicity. The liposome encapsulated PEP has promise as an antitumor agent, alone or in combination with inhibitors of DNA synthesis.

Double haploidentical hematopoietic stem cell transplantation results in successful engraftment of bone marrow from both donors without graft-versus-host or graft-versus-graft effects.

We established double-haploidentical (DH) hematopoietic stem cell transplantation (HSCT) murine models to explore competitive engraftment, graft-versus-graft effect and graft-versus-host disease (GVHD). T cell-depleted (TCD) bone marrow (BM) cells from B6SJF1 (donor 1 [D1]) and B6D2F1 (donor 2 [D2]) mice achieved >90% donor engraftment when transplanted into B6CBAF1 mice. B6CBAF1 recipients survived without evidence of GVHD when undergoing HSCT with TCD-BM from 2 haploidentical donors, D1 and D2. DH-HSCT recipients had significantly higher leukocyte and neutrophil counts than single-haploidentical HSCT recipients from either D1 or D2. DH recipients consistently showed successful mixed chimerism in both BM and spleen. Two other DH-HSCT models, B6D2F1 + C3D2F1→B6C3F1 and B6CBAF1 + B6SJLF1→B6D2F1, showed similar engraftment patterns. Low-dose T cell infusion from both D1 and D2 increased the degree of early engraftment of the respective donors in BM and spleen; however, this early engraftment pattern did not determine long-term engraftment dominance. In the long term, minimally engrafted D1 BM recovered and comprised >50% of all donor- derived B, T, and natural killer cells. We conclude that early BM engraftment is determined by donor T cell immunodominance, but long-term engraftment is related to the engraftment potential of stem cells after DH-HSCT.

Plitidepsin (Aplidin) is a potent inhibitor of diffuse large cell and Burkitt lymphoma and is synergistic with rituximab.

Plitidepsin (Aplidin), an antitumor agent of marine origin, presently is undergoing phase II/III clinical trials, and has shown promise for the treatment of lymphoma. Here, we describe the antitumor effects of plitidepsin alone and in combination with rituximab and investigated the effects of each drug and the combination on the cell cycle and mechanism of cell death. Several Diffuse Large Cell Lymphoma (DLCL) lines and Burkitt cell lines were tested for sensitivity to plitidepsin and rituximab. All DLCL and Burkitt lymphoma cell lines were inhibited by plitidepsin in nanomolar concentrations, while rituximab sensitivity varied among different cell lines. Ramos and the RL cell lines proved sensitive to rituximab and were used to test the effects of each of the two drugs. The two agents exhibited synergism at all tested concentrations. For in vivo studies, irradiated athymic nude mice were engrafted with the Ramos lymphoma. Treatment was initiated when the tumors were ~0.5 cm in diameter, and toxic and therapeutic effects were monitored. In the in vivo study, additive effects of the combined two drugs, was demonstrated without an increase in host toxicity. The in vitro synergy and the in vivo additive antitumor effects without an increase in host toxicity with two relatively non-marrow suppressive agents encourages further development of this combination for treatment of aggressive B-cell lymphomas.

Long-term culture-initiating cells (LTC-IC) produced from CD34+ cord blood cells with limiting dilution method.

OBJECTIVE: Even though much progress has been made in defining primitive hematologic cell phenotypes by using flow cytometry and clonogenic methods, the direct method for study of marrow repopulating cells still remains to be elusive. Long Term Culture-Initiating Cells (LTC-IC) are known as the most primitive human hematopoietic cells detectable by in vitro functional assays. METHODS: In this study, LTC-IC with limiting dilution assay was used to evaluate repopulating potential of cord blood stem cells. RESULTS: CD34 selections from cord blood were completed succesfully with magnetic beads (73,64%±9,12). The average incidence of week 5 LTC-IC was 1: 1966 CD34+ cells (range 1261-2906). CONCLUSION: We found that number of LTC-IC obtained from CD34+ cord blood cells were relatively low in numbers when compared to previously reported bone marrow CD34+ cells. This may be due to the lack of some transcription and growth factors along with some cytokines and chemokines released by accessory cells which are necessary for proliferation of cord blood progenitor/stem cells and it presents an area of interest for further studies.

Chemoprotection by transfer of resistance genes.

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.