Gemtuzumab ozogamicin (Mylotarg®; Pfizer/Wyeth-Ayerst Laboratories) is a recombinant, humanized anti-CD33 monoclonal antibody (IgG4 κ antibody hP67.6) covalently attached to the cytotoxic antitumor antibiotic calicheamicin (N-acetyl-γ-calicheamicin) via a bifunctional linker (4-(4-acetylphenoxy)butanoic acid). The drug has been studied for treatment of acute myeloid leukemia (AML), a bone marrow cancer.
In this antibody-drug-conjugate or ADC, the antibody binds to and is internalized by tumor cells expressing CD33 antigen (a sialic acid-dependent glycoprotein commonly found on the surface of leukemic blasts), thereby delivering the attached calicheamicin to CD33-expressing tumor cells.
Calicheamicins are potent anti-tumour antibiotics that were initially identified by their ability to damage DNA in screening tests. The anti-tumour mechanism of calicheamicin is thought to occur by binding to the minor groove in the DNA and producing site-specific double-strand breaks by forming p-benzene diradical. These double strand DNA breaks result in the inhibition of DNA synthesis leading in the death of the leukemic cell.
The hP67.6 antibody is produced by humanization of the anti-CD33 murine antibody hP67.6 by complementarity determining region grafting (CDR grafting). The resulting hP67.6 antibody is a genetically engineered human IgG4 kappa antibody, which contains sequences derived from the murine antibody in the antigen-binding region only to minimise human anti-murine antibody response. This IgG4 isotype was chosen because it has the longest circulating half-life of all isotypes and is least likely to participate in immune-mediated mechanisms such as complement fixation and antibody-dependent cellular toxicity.
In gemtuzumab ozogamicin the bifunctional 4-(4-acetylphenoxy)butanoic acid (AcBut) moiety provides attachment to surface-exposed lysines of the antibody through an amide bond. This forms an acyl hydrazone linkage with N-acetyl-γ-calicheamicin dimethyl hydrazide. On average, a drug loading (Drug to antibody ratio or DAR) of 2 to 3 molecules of calicheamicin per molecule of mAb can be achieved. Upon internalization of the ADC, the calicheamicin prodrug is released by hydrolysis of the hydrazone in the lysosomes of the CD33+ target cells.
Mode of action
Gemtuzumab ozogamicin is a humanized (CDR-grafted) IgG4 isotype monoclonal antibody against the CD33 antigen (hP67.6) that is conjugated to a cytotoxic agent N-acetyl gamma calicheamicin dimethyl hydrazide (NAc-gamma calicheamicin DMH) via the bifunctional AcBut linker.
The non-cytotoxic hP67.6 antibody binds to the CD33 antigen, is internalized and, following internalization, delivers the calicheamicin derivative to the inside of the leukemic cell. The CD33 antigen is expressed on the external surface of normal and leukemic myeloid cells, and leukemic blasts in more than 80% of patients with acute myeloid leukemia (AML), but not on normal precursor hematopoietic cells. The antibody linked to the conjugate is internalized into lysosomes, where acidification releases the NAc-gamma calicheamicin DMH moiety. The latter undergoes spontaneous reaction with reduced glutathione (GSH) within the cell, where it is activated and the anti-tumour effect can occur.
The hP67.6 antibody component of gemtuzumab ozogamicin was found to bind only to CD33+ cells and was non-cytotoxic in tissue culture. As a human monoclonal antibody of the IgG4 isotype class, neither complement-binding nor antibody-dependent cellular cytotoxicity (ADCC) was expected or observed in test animals.
- Calicheamicin-Conjugated Humanized Anti-CD33 Monoclonal Antibody
Additional code names
The drug was approved in May 2000 under the FDA’s accelerated approval program. Wyeth (now Pfizer) started a confirmatory, post approval, clinical trial in 2004. The trial was designed to determine whether adding gemtuzumab ozogamicin to standard chemotherapy demonstrated an improvement in clinical benefit (survival time) for patients with acute myeloid leukemia.
At initial approval, gemtuzumab ozogamicin was associated with a serious liver condition called veno-occlusive disease, which can be fatal. This rate has increased in the postmarket setting.
The post marketing clinical trial was discontinued early when researchers did not observe an improvement in clinical benefit in the randomized SWOG 106 study, which added 6 mg/m2 to 3 + 7 (3-day anthracycline plus 7-day cytarabine) in untreated patients age < 60 years.  In this trial, the researchers notice a greater number of deaths in the group of patients who received gemtuzumab ozogamicin compared with those receiving chemotherapy alone.
Based on these results, Pfizer voluntarily withdrew gemtuzumab ozogamicin from the market in mid-2010. As a result, the drug was no longer commercially available to new patients. Patients who, at the time, received the drug where able complete their therapy following consultation with their health care professional. Health care professionals instructed to inform all patients receiving gemtuzumab ozogamicin of the product’s potential safety risks. The withdraw made it a requirement that future use of gemtuzumab ozogamicin in the United States would require submission of an investigational new drug application to the U.S. Food and Drug Administration (FDA).
Following the withdrawal of the drug in the United States, gemtuzumab ozogamicin remained on the market in Japan where it continued to be approved for the treatment of patients with relapsed or refractory CD33-positive AML who are not considered candidates for other cytotoxic chemotherapy.
The drug has also been available to individual patients through Pfizer’s compassionate use programs. Due to the critical unmet need for patients with AML, there remained great interest among AML clinicians to evaluate gemtuzumab ozogamicin using different doses and different schedules. These independent investigators, with Pfizer’s support, conducted clinical trials that yielded more information on the efficacy and safety of gemtuzumab ozogamicin.
In januari 2017 Pfizer’s Biologics License Application (BLA; BLA761060) for gemtuzumab ozogamicin was accepted for filing by the FDA. A Marketing Authorization Application (MAA) for review by the European Medicines Agency (EMA) was validated in December 2016.
On September 1, 2017, the FDA (re)approved gemtuzumab ozogamicin. The approval included a lower recommended dose, a different schedule in combination with chemotherapy or on its own, and a new patient population.
Following the withdrawal in 201o, ongoing clinical trials with gemtuzumab ozogamicin confirmed benefit, suggesting that it may be important to reassess and reverse the decision to withdraw the drug. 
This conclusion was based on several investigator-led clinical trials, including ALFA-0701, AML-19 and MyloFrance-1.
The ALFA-0701 (NCT00927498) trial was a Phase III, multicenter, randomized, open-label study of 271 patients with newly-diagnosed de novo AML, using a new, lower fractionated dose of gemtuzumab ozogamicin. In this trial, conducted only in France, patients received gemtuzumab ozogamicin 3 mg/m2 on days 1, 4 and 7 in combination with conventional chemotherapy or chemotherapy alone.
CD33-positivity was not required for eligibility to participate in the trial. Treatment consisted of induction followed by two cycles of consolidation, and follow-up extended through 2 years from randomization.
The primary objective of the protocol was to evaluate the efficacy of adding gemtuzumab ozogamicin to standard chemotherapy in patients 50- 70 years old with AML.
The primary endpoint of the ALFA-0701 trial was event-free survival or EFS. Administering gemtuzumab ozogamicin (n=135) in addition to standard induction chemotherapy resulted in a significant improvement in EFS compared with chemotherapy alone (n=136) in patients with newly diagnosed AML. Event-free survival was 17.3 months for patients receiving gemtuzumab ozogamicin compared with 9.5 months for those receiving chemotherapy alone (HR = 0.56 [95% CI: (0.42, 0.76)]).
Secondary objectives of the trial included evaluation of rate of Complete Remission (CR) and Complete Remission CR with incomplete platelet recovery to >100 Gi/L (CRp). CR was defined as no circulating blasts, marrow blast cells < 5%, Hgb > 9g/dL, platelets >100 Gi/L, neutrophils > 1 Gi/L, and transfusion independence. Cumulative response rate, relapse-free survival, overall survival, and the safety profile of the combination were also examined.
Study AML-19 (NCT00091234) was a multicenter, randomized, open-label Phase III study comparing single agent gemtuzumab ozogamicin (n=118) to best supportive care (n=119) for elderly patients who could not tolerate other AML therapies.
As initial treatment, patients received gemtuzumab ozogamicin 6 mg/m2 on day 1 and gemtuzumab ozogamicin 3 mg/m2 on day 8. As continued treatment, patients without evidence of disease progression received gemtuzumab ozogamicin 2 mg/m2 on day 1 every 4 weeks.
The efficacy of gemtuzumab ozogamicin was established on the basis of a significant improvement in overall survival (OS). Median OS was 4.9 months for patients receiving gemtuzumab ozogamicin compared with 3.6 months for patients receiving best supportive care (HR=0.69 [95% CI: 0.53-0.90] [2-sided p=0.005]).
MyloFrance-1 was a Phase II, single-arm, open-label study of 57 adult patients in first relapse. Patients received single agent gemtuzumab ozogamicin 3mg/m2 on days 1, 4 and 7. The efficacy of gemtuzumab ozogamicin was established on the basis of complete remission (CR) rate and duration of remission. In the trial, 15 (26%; 95% CI: 16%-40%) patients achieved a complete remission (CR) and median relapse-free survival (RFS) was 11.6 months.
- Prescribing information gemtuzumab ozogamicin (Mylotarg®; Pfizer/Wyeth)
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First editorial review: July 19, 2015
Last editorial review: November 22, 2017
Information based on ClinicalTrials.gov (NIH/NCI) and other sources.
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