Gemtuzumab ozogamicin: first clinical experiences in children with relapsed/refractory acute myeloid leukemia treated on compassionate-use basis

Gemtuzumab ozogamicin: a review of its use in acute myeloid leukaemia

Gemtuzumab ozogamicin (Mylotarg) is a conjugate of a monoclonal antibody and calicheamicin, which targets the membrane antigen CD33 in CD33-positive acute myeloid leukaemia (AML) and, after cell internalisation, releases a derivative of the cytotoxic calicheamicin component. In the US, it is approved as monotherapy in patients aged > or =60 years with a first relapse of AML who are ineligible for other cytotoxic therapy. Monotherapy with gemtuzumab ozogamicin results in complete remission (CR) or CR with incomplete platelet recovery (CRp) in approximately =25% of adults (including those aged > or =60 years) with CD33-positive AML in first relapse. Preliminary data indicate a potential role for gemtuzumab ozogamicin as a component of induction or consolidation regimens in adults and, based on an early study, in the treatment of children with AML, although randomised, controlled studies are needed. Serious adverse events, notably hepatotoxicity, characterise its tolerability profile, but gemtuzumab ozogamicin is comparatively well tolerated by most patients. Gemtuzumab ozogamicin is a valuable new treatment option for patients aged > or =60 years with CD33-positive AML in first relapse for whom other cytotoxic chemotherapy is not considered appropriate; patients with a first CR (CR1) of >12 months are likely to have the best outcome.

Preliminary Results of the Safety of Immunotherapy with Gemtuzumab Ozogamicin following Reduced Intensity Allogeneic Stem Cell Transplant in Children with CD33+ Acute Myeloid Leukemia

PURPOSE

Myeloablative allogeneic stem cell transplantation (SCT) has been successful in the treatment of childhood acute myeloid leukemia (AML), but may be associated with significant toxicity and recurrent disease. Reduced-intensity allogeneic SCT may offer a less toxic approach to patients with AML. Targeted immunotherapy with gemtuzumab ozogamicin has been shown to be safe, well tolerated in children, and, as a single agent, gemtuzumab ozogamicin has induced responses in 30% of patients with recurrent CD33+ AML. There are no safety data with gemtuzumab ozogamicin post allogeneic SCT in children. Therefore, we explored the feasibility and toxicity of targeted immunotherapy following reduced-intensity allogeneic SCT in children with CD33+ AML.

EXPERIMENTAL DESIGN

Eight patients with CD33+ AML received a reduced-intensity allogeneic SCT following fludarabine 30 mg/m2 for 6 days and busulfan 3.2 mg/kg (<4 years, 4 mg/kg/d) for 2 days. Donor sources included six 6/6 HLA-matched related peripheral blood stem cells, one 6/6 sibling cord blood, and one 4/6 unrelated cord blood.

RESULTS

Day 30 and day 60 donor chimerisms in seven of eight evaluable patients were 96 +/- 2% (n = 7) and 94 +/- 3% (n = 6), respectively. Five of six patients (too early for one patient) received two doses of gemtuzumab ozogamicin and one patient received only one dose. After each dose, all patients developed grade 4 neutropenia, with recovery on median days 16 and 13, respectively, after dose 1 and dose 2. Grade 4 thrombocytopenia was only observed in 2 of 11 gemtuzumab ozogamicin courses. No patients have developed dose-limiting toxicity secondary to gemtuzumab ozogamicin.

CONCLUSIONS

The administration of gemtuzumab ozogamicin post reduced-intensity allogeneic SCT in children with average risk AML is feasible and well tolerated with minimal toxicity. The maximal tolerated dose has yet to be determined for gemtuzumab ozogamicin post reduced-intensity allogeneic SCT in children with CD33+ AML. Additional studies in a larger group of patients will be required to adequately assess the safety of this approach.

Treatment strategy and results in children treated on three Dutch Childhood Oncology Group acute myeloid leukemia trials

This report describes the long-term follow-up data of three consecutive Dutch Childhood Oncology Group acute myeloid leukemia (AML) protocols. A total of 303 children were diagnosed with AML, of whom 209 were eligible for this report. The first study was the AML-82 protocol. Results were inferior (5-year probability of overall survival (pOS) 31%) to other available regimes. Study AML-87 was based on the BFM-87 protocol, with prophylactic cranial irradiation in high-risk patients only, and without maintenance therapy. This led to a higher cumulative incidence of relapse than that reported by the Berlin-Frankfurt-Münster (BFM), but survival was similar (5-year pOS 47%), suggesting successful retrieval at relapse. The subsequent study AML-92/94 consisted of a modified BFM-93 protocol, that is, without maintenance therapy and prophylactic cranial irradiation. However, all patients were to be transplanted (auto- or allogeneic), although compliance was poor. Antileukemic efficacy was offset by an increase in the cumulative incidence of nonrelapse mortality, especially in remission patients, and survival did not improve (5-year pOS 44%). Our results demonstrate that outcome in childhood AML is still unsatisfactory, and that further intensification of therapy carries the risk of enhanced toxicity. Our patients are currently included in the MRC AML studies, based on the results of their AML 10 trial.

Outcome and long-term follow-up of alloreactive donor lymphocyte infusions given for relapse after myeloablative allogeneic hematopoietic stem cell transplantations (HSCT)

In order to study efficacy, toxicity and the long-term results of donor lymphocyte infusions (DLI), we retrospectively analyzed DLI given for relapse after conventional allogeneic hematopoietic stem cell transplantation (HSCT) in 30 patients with a median delay of 107.5 months after transplant and 58 months after DLI. After DLI, 15 patients established full donor chimerism, three patients developed grade III and one grade IV acute GVHD. A total of 15 patients achieved a disease response. Among the 14 patients with chronic myeloid leukemia (CML), 11 are alive at the last follow-up: five are in complete molecular response (CMR) and two in complete cytogenetic response (CCR) with no other intervention after DLI, three in CMR after imatinib mesylate given after DLI and one in complete hematological response after imatinib mesylate and reduced-intensity conditioning allogeneic SCT performed after DLI. At the time of the last follow-up, 19 (63%) patients died and 11 (37%) remain alive. The 3-year probability of survival for the entire population, CML patients and non-CML patients, was 60, 93, 62% after transplantation, and 48, 80 and 48% after DLI, respectively. A multivariate analysis demonstrated a significantly worse survival rate after transplantation for female recipients, advanced disease and acute leukemia before transplantation.

Pharmacokinetics of gemtuzumab ozogamicin as a single-agent treatment of pediatric patients with refractory or relapsed acute myeloid leukemia

Gemtuzumab ozogamicin is currently approved to treat CD33-positive acute myeloid leukemia (AML) in first relapse in patients older than age 60 years. The objective of this study was to characterize the pharmacokinetics of gemtuzumab ozogamicin in pediatric patients with relapsed or refractory AML. The study population comprised 29 subjects younger than age 18 with AML in first relapse. Dosages of 6, 7.5, and 9 mg/m(2) were administered during the study. Pharmacokinetic parameters were determined following each dose for hP67.6, total calicheamicin derivatives, and unconjugated calicheamicin derivatives. hP67.6 pharmacokinetic parameters had a consistent and statistically significant change between the first and second doses. Increases in AUC and decreases in both CL and V(ss) from the first dose to the second dose were consistent with those of the adult population. Changes between dose periods for total calicheamicin derivatives and unconjugated calicheamicin derivatives were consistent with those of hP67.6. Changes in pharmacokinetic parameters between dose periods are attributed to saturation of CD33 binding sites and diminished clearance resulting from a lower peripheral blast burden and antigen. Children receiving 9 mg/m(2) had the following hP67.6 pharmacokinetic parameters: C(max), 3.47+/-1.04 mg/L; AUC, 136 +/- 107 mg x h/L; CL, 0.12 +/- 0.15 L/h/m(2); V(ss), 6.5 +/- 5.5 L; and t(1/2), 64 +/- 44 h after their first dose. Mean pharmacokinetic values are similar to values reported in adults. Individual children demonstrated large intersubject variability, similar to adults. The pharmacokinetics of gemtuzumab ozogamicin in pediatric patients closely follow the profile and variability of adult patients.

Advances in childhood leukaemia: successful clinical-trials research leads to individualised therapy

In most cases, childhood leukaemia has a fetal origin, but multiple molecular events are required after birth for pre-leukaemic cells to progress to leukaemia. Cure rates for acute lymphoblastic leukaemia (ALL) now approach 80%. A high level of minimal residual disease detected by polymerase chain reaction in patients with ALL in remission has profound prognostic importance and is the focus of a major Australian study attempting to prevent relapse in these children. Greater awareness of the late effects of chemotherapy has led to changes in the treatment protocols for ALL, with improvement in neurocognitive outcomes and reduced rates of second malignancies. Pharmacogenetics is a new field of research that aims to enhance treatment efficacy by assessing the individual's metabolism of and response to chemotherapeutic agents. Targeted therapies currently being developed show some promise of being able to further improve cure rates. Adolescents with ALL have a better prognosis if treated with paediatric rather than adult protocols.

Gemtuzumab ozogamicin (Mylotarg) has therapeutic activity against CD33+ acute lymphoblastic leukaemias in vitro and in vivo

Gemtuzumab ozogamicin (GO) is a humanized anti-CD33 antibody conjugated with the cytotoxic drug calicheamicin and approved for the treatment of relapsed acute myeloid leukaemia. As approximately 18% of acute lymphoblastic leukaemias (ALL) are also CD33 positive, we have investigated the cytotoxic activity of GO on CD33+ ALL cells in vitro and in vivo. 10 ng/ml GO induced 30-95% inhibition of thymidine uptake and 30-70% cell death in four freshly isolated and one in vivo passaged CD33+ ALL-cell cultures. Furthermore, an in vivo model of a CD33+ ALL carrying the Philadelphia chromosome [t(9;22)] was established. 5 x 10(6) ALL-2 cells inoculated in the tail vein of severe combined immunodeficient mice engrafted into haematopoietic organs, reaching a mean of 70%, 61% and 69% human CD45+ cells in bone marrow, spleen and liver, respectively, at 35 d. To test the therapeutic activity of GO, 50 or 100 microg immunotoxin was inoculated i.p. on days 7, 11 and 15 following tumour-cell inoculation. GO treatment dramatically inhibited expansion of ALL-2 cells in all tested organs and increased survival of tumour-injected animals by 28-41 d, relative to controls. These data demonstrated that GO is active both in vitro and in vivo against CD33+ ALL cells.

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

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

Possibilities for tailored and targeted therapy in paediatric acute myeloid leukaemia

The clinical outcome of acute myeloid leukaemia (AML) in children has improved considerably using intensive chemotherapy and/or stem cell transplantation. This leads to cure in 50-70% of patients, and also results in significant morbidity and mortality. Hence, we need other ways to improve the cure rate. This review discusses possibilities for tailored therapy, reviewing in vitro cellular drug sensitivity data. The results provide suggestions regarding the adaptation of clinical protocols in certain AML subgroups, although further clinical studies will show whether this is effective. Secondly, we review type 1 genetic abnormalities (such as receptor tyrosine kinase mutations) that result in enhanced survival and proliferation of leukaemic cells, which can be detected in approximately 50% of paediatric AML samples, and are non-randomly associated with French-American-British type and cytogenetic subgroups. FLT3 internal tandem duplication is associated with poor clinical outcome, and may be used for risk-group stratification. The first results with small molecule inhibitors in adult AML do not suggest their use in children as yet. International collaboration is needed to further improve outcome by developing treatment protocols for subgroups of paediatric AML.

Antibody-targeted chemotherapy of acute myeloid leukemia using gemtuzumab ozogamicin (Mylotarg)

Acute myeloid leukemia (AML) is the most common type of acute leukemia in adults and accounts for 20% of pediatric leukemia. Although conventional chemotherapy induces clinical remissions in most patients with AML, recurrent leukemia represents the major obstacle to cure. Conventional chemotherapy reinduction is associated with limited efficacy and substantial toxicity. Chemotherapy specifically targeted to leukemic cells by monoclonal antibodies might enable patients to achieve remissions more safely than conventional approaches. After evaluating a series of phase II studies, the U.S. Food and Drug Administration approved Mylotarg (gemtuzumab ozogamicin) for the treatment of patients with CD33-positive AML in first relapse who are 60 years of age or older and who are not considered candidates for other types of cytotoxic chemotherapy. Among 277 adult patients with CD33-positive AML in first relapse, 26% experienced an overall response after Mylotarg monotherapy. Despite the fact that myelosuppression, hyperbilirubinemia, and elevated hepatic transaminases were commonly observed, the incidences of severe infections and mucositis were relatively low in comparison with conventional chemotherapeutic treatment. Preliminary reports in pediatric patients also report Mylotarg to be reasonably well tolerated. Recently, data from study regimens combining Mylotarg and conventional chemotherapy suggest an unusually high remission induction rate in de novo AML patients. Information assembled from prospective, ongoing studies in the United States and the United Kingdom should help us use this novel immunoconjugate in a safe and effective manner.