Intravenous Busulfan and Melphalan, Tacrolimus, and Short-Course Methotrexate Followed by Unmodified HLA-Matched Related or Unrelated Hematopoietic Stem Cell Transplantation for the Treatment of Advanced Hematologic Malignancies

Adding melphalan to fludarabine and a myeloablative dose of busulfan improved survival after allogeneic hematopoietic stem cell transplantation in a propensity score-matched cohort of hematological malignancies

Fludarabine and a myeloablative dose of busulfan (Flu/Bu4) can improve prognosis after allogeneic hematopoietic stem cell transplantation (HSCT) with melphalan (Mel). We investigated the prognostic impact of adding Mel to Flu/Bu4 by comparing between Flu/Bu4/Mel and Flu/Bu4 groups. This study included 846 propensity score (PS)-matched patients who received either Flu/Bu4/Mel (n = 423) or Flu/Bu4 (n = 423) from 2394 patients enrolled in a multicenter prospective registry, from January 2010 to December 2016. The primary endpoint (5-year overall survival [OS]), and the prognostic impact of adding Mel was evaluated using Cox regression analysis. The study population median age was 58 (interquartile 50-64) years and 61.0% were male. Patient characteristics were well-balanced between groups. Five-year OS was 34.2% (95% confidence interval [CI]: 27.3-41.1%) and 30.1% (24.8-35.6%) in the Flu/Bu4/Mel and Flu/Bu4 groups, respectively (log-rank P = 0.019). The adjusted hazard ratio of adding Mel was 0.77 (95% CI: 0.62-0.96) (P = 0.022) for the 5-year OS, and this attributed to a lower incidence of 5-year relapse (0.71, 0.56-0.90, P = 0.005) and relapse associated mortality (0.73, 0.57-0.95, P = 0.018). There was no statistical difference in 5-year non-relapse mortality between groups (log-rank P = 0.855). Flu/Bu4/Mel was associated with better 5-year OS compared to Flu/Bu4 in a PS-matched cohort after allogeneic HSCT.

In vitro and in vivo anti-leukemic activity of the peptidase-potentiated alkylator melflufen in acute myeloid leukemia

The novel aminopeptidase potentiated alkylating agent melflufen, was evaluated for activity in acute myeloid leukemia in a range of in vitro models, as well as in a patient derived xenograft study. All tested AML cell lines were highly sensitive to melflufen while melphalan was considerably less potent. In the HL-60 cell line model, synergy was observed for the combination of melflufen and cytarabine, an interaction that appeared sequence dependent with increased synergy when melflufen was added before cytarabine. Also, in primary cultures of AML cells from patients melflufen was highly active, while normal PBMC cultures appeared less sensitive, indicating a 7-fold in vitro therapeutic index. Melphalan, on the other hand, was only 2-fold more potent in the AML patient samples compared with PBMCs. Melflufen was equally active against non-malignant, immature CD34⁺ progenitor cells and a more differentiated CD34⁺ derived cell population (GM14), whereas the stem cell like cells were less sensitive to melphalan. Finally, melflufen treatment showed significant anti-leukemia activity and increased survival in a patient derived xenograft of AML in mice. In conclusion, melflufen demonstrates high and significant preclinical activity in AML and further clinical evaluation seem warranted in this disease.

Personalizing Busulfan-Based Conditioning: Considerations from the American Society for Blood and Marrow Transplantation Practice Guidelines Committee

The Practice Guidelines Committee of the American Society of Blood or Marrow Transplantation (ASBMT) sought to develop an evidence-based review about personalizing busulfan-based conditioning. The Committee sought to grade the relevant published studies (June 1, 2008 through March 31, 2016) according to criteria set forth by the Steering Committee for Evidence Based Reviews from ASBMT. Unfortunately, the published literature was too heterogeneous and lacked adequately powered and sufficiently controlled studies for this to be feasible. Despite this observation, the continued interest in this topic led the Practice Guidelines Committee to develop a list of most frequently asked questions (FAQs) regarding personalized busulfan dosing. This "Considerations" document is a list of these FAQs and their responses, addressing topics of practical relevance to hematopoietic cell transplantation clinicians.

A Novel Reduced-Toxicity Myeloablative Conditioning Regimen Using Full-Dose Busulfan, Fludarabine, and Melphalan for Single Cord Blood Transplantation Provides Durable Engraftment and Remission in Nonremission Myeloid Malignancies

A pilot study of a novel, reduced-toxicity, myeloablative conditioning regimen using intravenous busulfan 12.8 mg/kg, fludarabine 180 mg/m(2), and melphalan 80 mg/m(2) for single cord blood transplantation (CBT) was conducted at our institution. Fifty-one patients with myeloid malignancies not in remission were included in this study. Their median age was 59 years (range, 19 to 70 years), with a median hematopoietic cell transplantation-specific comorbidity index score of 3. With a median observation period of 39.6 months (range, 24.3 to 90.8 months) among the survivors, overall survival and progression-free survival at 2 years were both 54.9%. Forty-six of 51 achieved neutrophil engraftment at a median of 19.5 days (range, 13 to 38 days) after transplantation, with a cumulative incidence of 90.2%. No patient developed graft rejection in this study. All patients who achieved engraftment showed hematological complete remission with complete donor chimerism. Eleven patients relapsed at a median of 4.9 months (range, .5 to 26.7 months). Cumulative incidences of nonrelapse mortality (NRM) at 100 days and 2 years were 11.8% and 25.5%, respectively. In conclusion, the present results show that the novel conditioning regimen for single CBT provided durable engraftment and remission with acceptable NRM leading to excellent survival, even for a relatively older population with myeloid malignancies not in remission.

Stem cell transplantation in adults with acute myelogenous leukemia, normal cytogenetics, and the FLT3-ITD mutation

Following chemotherapy, patients with acute myelogenous leukemia (AML), normal cytogenetics, and the FLT3-ITD mutation have shorter survival compared to wild type (WT) patients. To determine the role of stem cell transplantation (SCT), we performed a retrospective review of adult patients with AML who underwent SCT at our center between January 1, 2007 and December 31, 2011. Of the 200 patients transplanted, 79 patients (40%) had normal cytogenetics; of these, 17 (22%) had the FLT3-ITD mutation, 35 were WT (44%), and 27 (34%) did not have FLT3 testing performed. Clinical characteristics were similar in each group. At four years, overall survival (OS) between FLT3 positive and WT groups was similar: 0.54 (95% CI 0.29-0.75) versus 0.73 (95% CI 0.53-0.99), p=0.18. Relapse rates were also similar, 0.09 (95% CI 0.02-0.21) versus 0.12 (95% CI: 0.02-0.32); p=0.67. We conclude that SCT can abrogate the poor prognosis in FLT3-ITD positive patients.

The graft-versus-myeloma effect: chronic graft-versus-host disease but not acute graft-versus-host disease prolongs survival in patients with multiple myeloma receiving allogeneic transplantation

We conducted a study of patients with multiple myeloma (MM) undergoing allogeneic transplantation to evaluate outcome parameters. Fifty-seven consecutive patients with MM received an allogeneic transplantation between 2004 and 2011 at our institution. Patients who had received at least 1 prior autologous transplantation were included. Twenty-six patients underwent allogeneic transplantation for consolidation after a response to their first autograft, and 30 patients received an allogeneic transplantation as salvage therapy. Donor source was evenly distributed between related and unrelated. The median follow-up was 52 months. Thirty-two (57.1%) patients achieved a complete response (CR). At 5 years, 49.2% of all patients were in CR. Sixteen patients received either donor lymphocyte infusions or immune suppression withdrawal for disease progression, with a 62.5% response rate. The 5-year overall survival (OS) for all patients was 59%. The 5-year OS for the 30 patients in the consolidation group was 82% compared with 38% for those in the salvage group. In multivariate analysis, 3 factors remained significantly associated with OS. These include being in the salvage group (hazard ratio [HR], 4.05; P = .0196), acute graft-versus-host disease (aGVHD) (HR, 2.99; P = .034), and chronic graft-versus-host disease (cGVHD), which was highly protective, with a 5-year OS of 78.8% for patients with cGVHD versus 42.6% for patients without cGVHD (HR .17, P = .008). Our data show that allogeneic transplantation for MM can lead to sustained remissions. aGVHD is significantly deleterious to OS and progression-free survival, whereas cGVHD is strongly favorable, supporting an important role for the graft-versus-myeloma effect.

Intravenous BU plus Mel: an effective, chemotherapy-only transplant conditioning regimen in patients with ALL

We investigated the administration of i.v. BU combined with melphalan (Mel) in patients with ALL undergoing allogeneic hematopoietic SCT. Forty-seven patients with a median age of 33 years (range 20-61) received a matched sibling (n=27) or matched unrelated donor transplant (n=20) for ALL in first CR (n=26), second CR (n=13), or with more advanced disease (n=8). BU was infused daily for 4 days, either at a fixed dose of 130 mg/m² (5 patients) or using pharmacokinetic (PK) dose adjustment (42 patients), to target an average daily area-under-the-curve (AUC) of 5000 μmol/min, determined by a test dose of i.v. BU at 32 mg/m². This was followed by a rest day, then two daily doses of Mel at 70 mg/m². Stem cells were infused on the following day. The 2-year OS, PFS and non-relapse mortality (NRM) rates were 35% (95% confidence interval (CI), 23-51%), 31% (95% CI, 21-48%) and 37% (95% CI, 23-50%), respectively. Acute NRM at 100 days was favorable at 12% (95% CI, 5-24%); however, the 2-year NRM was significantly higher for patients older than 40 years, 58% vs 20%, mainly due to GVHD.

Fifty years of melphalan use in hematopoietic stem cell transplantation

Melphalan remains the most widely used agent in preparative regimens for hematopoietic stem cell transplantation (SCT). From its initial discovery more than 50 years ago, it has been gradually incorporated in the conditioning regimens for both autologous and allogeneic transplantations because of its myeloablative properties and broad antitumor effects as a DNA alkylating agent. Melphalan remains the mainstay conditioning for multiple myeloma and lymphomas, and it has been used successfully in preparative regimens of a variety of other hematological and nonhematological malignancies. The addition of newer agents to conditioning, such as bortezomib or lenalidomide for myeloma or clofarabine for myeloid malignancies, may improve antitumor effects for transplantation, whereas melphalan in combination with alemtuzumab may represent a backbone for future cellular therapy because of reliable engraftment and low toxicity profile. This review summarizes the development and the current use of this remarkable drug in hematopoietic SCT.

Allogeneic hematopoietic stem cell transplantation for pediatric patients with treatment-related myelodysplastic syndrome or acute myelogenous leukemia

The development of treatment-related myelodysplastic syndrome (tMDS) or treatment-related acute myelogenous leukemia (tAML) is a complication that can occur after chemotherapy or radiation therapy. Eighteen patients with a previous malignancy treated at our institution and three patients with a nonmalignant primary tumor received an allogeneic hematopoietic stem cell transplant (HSCT) on the pediatric bone marrow (BM) transplantation service for the treatment of tMDS/tAML over a 15-year period. Five patients proceeded to HSCT without induction chemotherapy. Fourteen patients received high-dose cytarabine according to the Capizzi II regimen as first-line induction therapy with 13 of them achieving complete remission (CR) or refractory anemia (RA) with persistent cytogenetic abnormalities after this treatment. Two patients received an anthracycline-based induction therapy. Conditioning regimens were selected according to previous therapies: 11 patients received busulfan-melphalan-fludarabine (BU-MEL-FLU), which consisted of busulfan (0.8 mg/kg/dose every 6 hours ×10 doses), melphalan (70 mg/m(2)/dose × two doses), and fludarabine (25 mg/m(2)/dose × five doses) for cytoreduction; three patients received a total body irradiation (TBI)-containing regimen; seven patients received myeloablative regimens containing busulfan and/or melphalan and/or thiotepa with doses modified for organ toxicity. Sixteen patients received T cell-depleted (TCD) grafts; four patients received unmodified grafts; one patient received a double-unit cord blood transplantation (DUCBT). Donors included HLA-matched (n = 9), or mismatched (n = 3) related donors, or HLA-matched (n = 4), or mismatched (n = 4) unrelated donors, or DUCBT (n = 1). Disease status at the time of HSCT was: morphologic and cytogenetic CR (n = 12); RA with positive cytogenetics (n = 6); and refractory disease (n = 3). With a median follow-up of 5.9 years (2.2-15.7 years), the 5-year overall survival (OS) and disease-free survival (DFS) rates for the entire group were 61.1% with 12 patients alive without evidence of either primary disease or tMDS/tAML. The OS and DFS rate for the 11 patients who received the BU-MEL-FLU cytoreduction with TCD grafts was 54.5%. DFS was 65.7% for patients in RA or CR at HSCT compared with 0% for patients with >5% residual marrow blasts (P = .015). Nine patients died; the cause of death was relapse of MDS/AML (n = 4) or primary disease (n = 2), graft-versus-host disease (GVHD; n = 2), and infection (n = 1). Four patients developed grade II to IV acute GVHD. One patient developed localized chronic GVHD. Our results suggest that the strategy of induction with high-dose cytarabine therapy followed by allogeneic stem cell transplantation improves the overall outcome for patients with tMDS/tAML. In addition, the use of a TCD transplantation with BU-MEL-FLU as cytoreduction may decrease the toxicity of transplantation in heavily pretreated patients without an increase in relapse rate.

Stem cells: a model for screening, discovery and development of drugs

The identification of normal and cancerous stem cells and the recent advances made in isolation and culture of stem cells have rapidly gained attention in the field of drug discovery and regenerative medicine. The prospect of performing screens aimed at proliferation, directed differentiation, and toxicity and efficacy studies using stem cells offers a reliable platform for the drug discovery process. Advances made in the generation of induced pluripotent stem cells from normal or diseased tissue serves as a platform to perform drug screens aimed at developing cell-based therapies against conditions like Parkinson’s disease and diabetes. This review discusses the application of stem cells and cancer stem cells in drug screening and their role in complementing, reducing, and replacing animal testing. In addition to this, target identification and major advances in the field of personalized medicine using induced pluripotent cells are also discussed.

Reduced late mortality risk contributes to similar survival after double-unit cord blood transplantation compared with related and unrelated donor hematopoietic stem cell transplantation

Cord blood transplantation (CB-T) is increasingly used as a treatment alternative for hematologic malignancies. However, how CB-T compares to related (RD-T) and unrelated donor transplantation (URD-T) is not established. We compared survival of 75 double-unit CB-T, 108 RD-T, and 184 URD-T recipients who received transplants over the same period for the treatment of hematologic malignancies. Patients had similar ages and disease risk, and a similar percentage had acute leukemia. The incidence of day 180 transplant-related mortality (TRM) of 21% (95% confidence interval [CI]: 12-31) after CB-T was higher than that of RD-T recipients. However, this was compensated for by a low risk of TRM after day 180, and a relatively low incidence of relapse. Hence, the 2-year progression-free survival (PFS) of 55% (95% CI: 45-68) after CB-T was similar to that after RD-T or URD-T (P = .573). In multivariate analysis, donor source had no influence on PFS, with the only significant factors being recipient age and disease risk. In a subanalysis of 201 patients with acute leukemia, CB-T, RD-T, and URD-T recipients also had similar 2-year disease-free survival (P = .482). These data provide strong support for the further investigation of double-unit CB grafts as an alternative hematopoietic stem cell source.

TBI and melphalan followed by allogeneic hematopoietic SCT in children with advanced hematological malignancies

We evaluated the efficacy and safety of the conditioning regimen that consisted of TBI and melphalan (L-PAM), followed by hematopoietic SCT (HSCT) in 23 children with advanced hematological malignancies. The median age at HSCT was 9 (range, 2-15) years. The underlying diseases were ALL in 16 patients (5 in CR2, 3 in CR3, 6 in relapse (RP) and 2 in induction failure (IF)), AML in 4 patients (3 in RP and 1 in IF) and non-Hodgkin's lymphoma in 3 patients (1 in CR3, 1 in CR4 and 1 in RP). The stem cell sources were BM for 19 patients and cord blood for 4 patients. All patients received the conditioning regimen that consisted of TBI 12 or 13.2 Gy and L-PAM 210 mg/m(2). In all, 22 patients engrafted on the median of day 16 (range, 10-23). The regimen was well tolerated and common regimen-related toxicities (RRTs) included grade II stomatitis and grade I hepatic toxicity. The cumulative incidences of RP and TRM were 47.6 and 21.5%, respectively. At a median follow-up of 24.4 months, the probability of disease-free survival was 41.0%. The regimen may provide sufficient anti-leukemic effect without increased RRT for advanced pediatric hematological malignancies.

Clofarabine ± fludarabine with once daily i.v. busulfan as pretransplant conditioning therapy for advanced myeloid leukemia and MDS

Although a combination of i.v. busulfan (Bu) and fludarabine (Flu) is a safe, reduced-toxicity conditioning program for acute myelogenous leukemia/myelodysplastic syndromes (AML/MDS), recurrent leukemia posttransplantation remains a problem. To enhance the conditioning regimen's antileukemic effect, we decided to supplant Flu with clofarabine (Clo), and assayed the interactions of these nucleoside analogs alone and in combination with Bu in Bu-resistant human cell lines in vitro. We found pronounced synergy between each nucleoside and the alkylator but even more enhanced cytotoxic synergy when the nucleoside analogs were combined prior to exposing the cells to Bu. We then designed a 4-arm clinical trial in patients with myeloid leukemia undergoing allogeneic stem cell transplantation (allo-SCT). Patients were adaptively randomized as follows: Arm I-Clo:Flu 10:30 mg/m(2), Arm II-20:20 mg/m(2), Arm III-30:10 mg/m(2), and Arm IV-single-agent Clo at 40 mg/m(2). The nucleoside analog(s) were/was infused over 1 hour once daily for 4 days, followed on each day by Bu, infused over 3 hours to a pharmacokinetically targeted daily area under the curve (AUC) of 6000 μMol-min ± 10%. Fifty-one patients have been enrolled with a minimum follow-up exceeding 100 days. There were 32 males and 19 females, with a median age of 45 years (range: 6-59). Nine patients had chronic myeloid leukemia (CML) (BC: 2, second AP: 3, and tyrosine-kinase inhibitor refractory first chronic phase [CP]: 4). Forty-two patients had AML: 14 were induction failures, 8 in first chemotherapy-refractory relapse, 7 in untreated relapse, 3 in second or subsequent relapse, 4 were in second complete remission (CR), and 3 in second CR without platelet recovery (CRp), 2 were in high-risk CR1. Finally, 1 patient was in first CRp. Graft-versus-host disease (GVHD) prophylaxis was tacrolimus and mini-methorexate (MTX), and those who had an unrelated or 1 antigen-mismatched donor received low-dose rabbit-ATG (Thymoglobulin™). All patients engrafted. Forty-one patients had active leukemia at the time of transplant, and 35 achieved CR (85%). Twenty of the 42 AML patients and 5 of 9 CML patients are alive with a projected median overall survival (OS) of 23 months. Marrow and blood (T cell) chimerism studies at day +100 revealed that both in the lower-dose Clo groups (groups 1+2) and the higher-dose Clo groups (groups 3+4), the patients had a median of 100% donor (T cell)-derived DNA. There has been no secondary graft failure. In the first 100 days, 1 patient died of pneumonia, and 1 of liver GVHD. We conclude that (1) Clo ± Flu with i.v. Bu as pretransplant conditioning is safe in high-risk myeloid leukemia patients; (2) clofarabine is sufficiently immunosuppressive to support allo-SCT in myeloid leukemia; and (3) the median OS of 23 months in this high-risk patient population is encouraging. Additional studies to evaluate the antileukemic efficacy of Clo ± Flu with i.v. Bu as pretransplant conditioning therapy are warranted.

Busulfan in hematopoietic stem cell transplantation

The development of intravenous busulfan (Bu) and its incorporation in the preparative regimens for allogeneic stem cell transplantation has changed transplantation for myelogenous malignancies. Bypassing the oral route to achieve 100% bioavailability translated into improved control over drug administration, with increased safety and reliability of generating therapeutic Bu levels, maximizing antileukemic efficacy. Bu-nucleoside analog-based conditioning chemotherapy, thus far represented by fludarabine (Flu), is becoming the conditioning chemotherapy regimen of choice for patients with acute myelogenous leukemia (AML) at many transplant centers. The use of busulfan Bu-based conditioning is extending rapidly also to hematopoietic stem cell transplantation (HSCT) for lymphoid malignancies, genetic diseases, and umbilical cord blood transplantation.

Recent progress on tissue-resident adult stem cell biology and their therapeutic implications

Recent progress in the field of the stem cell research has given new hopes to treat and even cure diverse degenerative disorders and incurable diseases in human. Particularly, the identification of a rare population of adult stem cells in the most tissues/organs in human has emerged as an attractive source of multipotent stem/progenitor cells for cell replacement-based therapies and tissue engineering in regenerative medicine. The tissue-resident adult stem/progenitor cells offer the possibility to stimulate their in vivo differentiation or to use their ex vivo expanded progenies for cell replacement-based therapies with multiple applications in human. Among the human diseases that could be treated by the stem cell-based therapies, there are hematopoietic and immune disorders, multiple degenerative disorders, such as Parkinson's and Alzheimer's diseases, type 1 or 2 diabetes mellitus as well as eye, liver, lung, skin and cardiovascular disorders and aggressive and metastatic cancers. In addition, the genetically-modified adult stem/progenitor cells could also be used as delivery system for expressing the therapeutic molecules in specific damaged areas of different tissues. Recent advances in cancer stem/progenitor cell research also offer the possibility to targeting these undifferentiated and malignant cells that provide critical functions in cancer initiation and progression and disease relapse for treating the patients diagnosed with the advanced and metastatic cancers which remain incurable in the clinics with the current therapies.

Once-daily intravenous busulfan with therapeutic drug monitoring compared to conventional oral busulfan improves survival and engraftment in children undergoing allogeneic stem cell transplantation

Because of intra- and interindividual variability, bioavailability, and pharmacokinetics of busulfan (Bu) in children, oral busulfan without therapeutic drug monitoring (TDM) is assumed to be associated with higher graft failure rates as well as higher toxicity (eg, veno-occlusive disease [VOD]). This study compares the outcome of hematopoietic stem cell transplantation (HSCT) of 2 groups: 1) 30 patients who received myeloablation with once-daily intravenous (i.v.) dose-targeted busulfan (BUdtIV) based on TDM and 2) 30 patients who received the current practice of untargeted oral busulfan (BUPO). Patients received a 3-hour infusion of Bu at a first dose of 120 mg/m(2) (age >or=1 year) or 80 mg/m(2) (<1 year), or BUPO 1 mg/kg 4 times daily. Both regimens were continued for 4 days. The target area under the curve (AUC) was defined as 17,500 microg *h/l. BUdtIV resulted in higher event-free survival (EFS) and survival rates compared to BUPO (EFS: 30% versus 83%, P < .001, survival: 53% versus 83%, P = .016). BUdtIV was associated with more cases of VOD. TDM was feasible in routine clinical practice. The results show that i.v. Bu using TDM is preferable over oral Bu in children undergoing allogeneic stem cell transplantation, especially in those at high risk for graft failure/relapse.

Recent advances on the molecular mechanisms involved in the drug resistance of cancer cells and novel targeting therapies

This review summarizes the recent knowledge obtained on the molecular mechanisms involved in the intrinsic and acquired resistance of cancer cells to current cancer therapies. We describe the cascades that are often altered in cancer cells during cancer progression that may contribute in a crucial manner to drug resistance and disease relapse. The emphasis is on the implication of ATP-binding cassette (ABC) multidrug efflux transporters in drug disposition and antiapoptotic factors, including epidermal growth factor receptor cascades and deregulated enzymes in ceramide metabolic pathways. The altered expression and activity of these signaling elements may have a critical role in the resistance of cancer cells to cytotoxic effects induced by diverse chemotherapeutic drugs and cancer recurrence. Of therapeutic interest, new strategies for reversing the multidrug resistance and developing more effective clinical treatments against the highly aggressive, metastatic, and recurrent cancers, based on the molecular targeting of the cancer progenitor cells and their further differentiated progeny, are also described.

Stem cells: a revolution in therapeutics-recent advances in stem cell biology and their therapeutic applications in regenerative medicine and cancer therapies

Basic and clinical research accomplished during the last few years on embryonic, fetal, amniotic, umbilical cord blood, and adult stem cells has constituted a revolution in regenerative medicine and cancer therapies by providing the possibility of generating multiple therapeutically useful cell types. These new cells could be used for treating numerous genetic and degenerative disorders. Among them, age-related functional defects, hematopoietic and immune system disorders, heart failures, chronic liver injuries, diabetes, Parkinson's and Alzheimer's diseases, arthritis, and muscular, skin, lung, eye, and digestive disorders as well as aggressive and recurrent cancers could be successfully treated by stem cell-based therapies. This review focuses on the recent advancements in adult stem cell biology in normal and pathological conditions. We describe how these results have improved our understanding on critical and unique functions of these rare sub-populations of multipotent and undifferentiated cells with an unlimited self-renewal capacity and high plasticity. Finally, we discuss some major advances to translate the experimental models on ex vivo and in vivo expanded and/or differentiated stem cells into clinical applications for the development of novel cellular therapies aimed at repairing genetically altered or damaged tissues/organs in humans. A particular emphasis is made on the therapeutic potential of different tissue-resident adult stem cell types and their in vivo modulation for treating and curing specific pathological disorders.