Introduction


Acute myeloid leukemia (AML) remains one of the most biologically heterogeneous hematologic malignancies. Despite advances in molecular characterization and targeted therapies, AML continues to be associated with substantial morbidity and mortality, particularly among older adults. Historically, treatment relied primarily on intensive cytotoxic chemotherapy, often consisting of anthracycline- and cytarabine-based induction regimens followed by consolidation therapy or hematopoietic stem cell transplantation (HSCT).

Over the past two decades, however, treatment paradigms have shifted toward precision medicine. The emergence of targeted therapies directed against FLT3, IDH1, IDH2, BCL-2, menin, and CD33 has fundamentally changed AML management. Among these innovations, antibody-drug conjugates (ADCs) represent a unique therapeutic platform that combines the specificity of monoclonal antibodies with the potency of highly cytotoxic payloads.

Gemtuzumab ozogamicin (Mylotarg®; Pfizer/Wyeth) occupies a special place in oncology history as the first ADC approved for cancer treatment. Although its development was marked by initial enthusiasm, subsequent withdrawal, and eventual reapproval, gemtuzumab ozogamicin ultimately became one of the most important proof-of-concept agents in ADC development.

Today, gemtuzumab ozogamicin remains an important treatment option for selected patients with CD33-positive AML and continues to provide valuable lessons regarding biomarker-driven treatment, toxicity management, and optimization of ADC dosing strategies.


CD33 Biology and Therapeutic Rationale

The Biology of CD33


Gemtuzumab ozogamicin 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 the treatment of acute myeloid leukemia (AML), a bone marrow cancer. (see: drug description)

Gemtuzumab ozogamicin was developed in a collaboration between Wyeth-Ayerst Research (now part of Pfizer) and Celltech Chiroscience* with the goal of using Celltech’s antibody-humanization technology and Wyeth-Ayerst’s calicheamicin-conjugation technology.

Target: CD33

CD33, also known as Siglec-3 (sialic acid-binding immunoglobulin-like lectin-3), is a transmembrane glycoprotein expressed on cells of the myeloid lineage. The molecule functions as an inhibitory receptor that regulates immune signaling and cellular differentiation. CD33 expression begins during early myeloid development and remains present on:
  • Myeloblasts
  • Monocytes
  • Mature myeloid cells
  • Leukemic blasts in most AML cases
Importantly, CD33 is generally absent from pluripotent hematopoietic stem cells, thereby preserving normal hematopoietic regeneration following treatment.

Numerous studies have demonstrated CD33 expression in approximately 85% to 90% of AML cases, making it one of the most broadly expressed therapeutic targets in the disease.

Why CD33 Is an Attractive Therapeutic Target

Several characteristics make CD33 particularly suitable for ADC therapy: High Prevalence of Expression Unlike many molecular targets restricted to specific AML subgroups, CD33 is expressed across diverse cytogenetic and molecular subtypes.

Rapid Internalization Following antibody binding, CD33 undergoes efficient receptor-mediated endocytosis. This characteristic is particularly important because ADCs rely on internalization to deliver cytotoxic payloads intracellularly.

Relative Absence of Stem Cells The lack of expression on normal hematopoietic stem cells provides a therapeutic window that allows bone marrow recovery following treatment.

Stable Expression During Disease Evolution Many AML cases maintain CD33 expression throughout disease progression and relapse, supporting use across multiple treatment settings.

Antibody-Drug Conjugates: A Brief Overview

Antibody-drug conjugates (ADC) consist of three essential components:
  • Monoclonal antibody
  • Linker technology
  • Cytotoxic payload

How do ADCs work?

The antibody serves as a homing device that selectively recognizes target antigens on cancer cells. The linker connects the antibody to the payload and must balance stability in circulation with efficient intracellular release. The payload provides the cytotoxic activity responsible for cancer cell death.

The success of an ADC depends on optimizing all three components. Gemtuzumab ozogamicin was among the earliest attempts to integrate these principles into a clinically useful therapeutic platform.

Mechanism of Action of Gemtuzumab Ozogamicin

Antibody Component

Gemtuzumab Ozogamicin contains a humanized anti-CD33 IgG4 monoclonal antibody that selectively binds CD33-expressing leukemic cells. Following administration, the antibody circulates through the bloodstream and binds to CD33 on AML blasts. Binding initiates rapid internalization of the antibody-antigen complex.

Linker Technology

The antibody is conjugated to a derivative of calicheamicin via a hydrolyzable linker. After internalization, lysosomal degradation and intracellular processing facilitate the release of the cytotoxic payload. Although the linker technology used in MYLOTARG differs from more sophisticated modern ADC platforms, it represented a major technological advance at the time of development.

Calicheamicin Payload

Calicheamicin is an extraordinarily potent DNA-damaging agent derived from Micromonospora echinospora. Once released inside leukemic cells, calicheamicin binds to DNA and induces double-strand DNA breaks. This damage activates apoptotic pathways leading to cell death. Unlike conventional chemotherapy, which exposes both malignant and normal tissues to cytotoxic agents, ADC-mediated delivery concentrates drug activity within target cells.

Historical Development of Gemtuzumab Ozogamicin

Early Clinical Development

Interest in CD33-targeted therapy emerged during the 1980s and 1990s as investigators sought more selective approaches to AML treatment. Preclinical studies demonstrated:
  • High CD33 expression on AML blasts.
  • Effective antibody binding.
  • Efficient internalization.
  • Potent cytotoxic activity when linked to calicheamicin.
These findings provided the foundation for clinical development.

Accelerated FDA Approval

In May 2000, gemtuzumab ozogamicin received accelerated approval from the FDA for older adults with relapsed CD33-positive AML who were not considered candidates for intensive chemotherapy. This approval represented a landmark event because it marked the first ADC approved for cancer treatment. At the time, response rates observed in relapsed AML generated considerable excitement regarding the potential of targeted drug delivery.

Withdrawal from the Market

Despite initial enthusiasm, subsequent studies raised concerns regarding safety and efficacy. The SWOG S0106 trial (NCT00085709) evaluated the addition of gemtuzumab ozogamicin to frontline induction therapy. Unexpected findings included:
  • Increased early mortality.
  • Greater toxicity.
  • Lack of improvement in complete remission rates.
Particularly concerning was the emergence of hepatic veno-occlusive disease (VOD), also known as sinusoidal obstruction syndrome (SOS). VOD became recognized as one of the most serious toxicities associated with gemtuzumab ozogamicin. Based on these concerns, Pfizer voluntarily withdrew gemtuzumab ozogamicin from the market in 2010. For many observers, the withdrawal raised broader questions regarding the future viability of ADC technology.

The Rebirth of Gemtuzumab Ozogamicin

Understanding the Problem

Subsequent analyses suggested that the problem was not necessarily the target or payload but rather the dosing strategy. Earlier studies employed relatively high single doses that produced excessive toxicity. Investigators hypothesized that fractionated dosing could maintain efficacy while reducing toxicity.

Development of Fractionated Dosing

New clinical studies evaluated lower doses administered over multiple treatment days. This strategy offered several advantages:
  • Lower peak drug exposure.
  • Improved tolerability.
  • Reduced hepatic toxicity.
  • Preservation of antileukemic activity.
The approach ultimately transformed the therapeutic index of gemtuzumab ozogamicin.

The ALFA-0701 Trial

The ALFA-0701 (NCT00927498) trial was a Phase 3, 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 standard induction (conventional) chemotherapy consisting of daunorubicin and cytarabine with or without fractionated gemtuzumab ozogamicin 3 mg/m2 on days 1, 4, and 7, or chemotherapy alone. The pivotal study enrolled patients aged 50 to 70 years with newly diagnosed AML. The study demonstrated:
  • Significant improvement in event-free survival.
  • Reduced relapse risk.
  • Improved relapse-free survival.
  • Acceptable toxicity profile using fractionated dosing.
Long-term follow-up confirmed durable benefit and provided the strongest evidence supporting reintroduction of the drug into clinical practice.

AML-19: Older Adults Unsuitable for Intensive Therapy

AML-19 evaluated gemtuzumab ozogamicin in patients considered unsuitable for intensive induction chemotherapy. The 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 at 6 mg/m2 on day 1 and at 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 by 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]) This study addressed an important unmet need because older AML patients frequently have limited treatment options. The trial demonstrated meaningful clinical benefit and helped establish gemtuzumab ozogamicin as a treatment option beyond traditional intensive chemotherapy settings.

AAML0531: Pediatric AML

The Children's Oncology Group AAML0531 trial extended the evaluation of gemtuzumab ozogamicin into pediatric AML. Addition of gemtuzumab ozogamicin to chemotherapy significantly reduced relapse risk and improved event-free survival. These findings supported subsequent pediatric indications and reinforced the role of CD33-targeted therapy across age groups.

Lessons Learned from Gemtuzumab Ozogamicin Development

Gemtuzumab Ozogamicin was the first antibody-drug conjugate (ADC) approved by the FDA, serving as a model for the evolution of targeted cancer therapy. Its trajectory—from accelerated approval in 2000, withdrawal from the market in 2010, and re-approval in 2017—provides valuable insights into the development of modern oncology drugs. The key Lessons learned include:

Dosing and Scheduling Matters

Optimization of dosing schedules can dramatically alter an ADC's therapeutic index. The original high, single-dose regimen (9 mg/m²) led to significant toxicity, particularly hepatic veno-occlusive disease (VOD). The 2017 re-approval demonstrated that fractionated, lower-dose schedules enhance efficacy and greatly reduce liver toxicity.

Antigen Dynamics Are Crucial

Initial assumptions held that a 9 mg/m² dose would saturate all CD33 binding sites. Later studies showed that CD33 receptors on acute myeloid leukemia (AML) blasts are rapidly re-expressed within 72 hours. Fractionated dosing ensures ongoing drug availability to target these newly expressed receptors.

Target Selection Matters

CD33 remains one of the most validated targets in AML.

Confirmatory Trials Are Essential

Drugs granted accelerated approval must undergo rigorous post-marketing studies. For Gemtuzumab, a Phase III trial (SWOG-0106) did not show a survival advantage and reported higher VOD rates, prompting withdrawal. This underscores the limitations of relying solely on early-phase data for dosing decisions.

Benefit from Combination Therapy

Gemtuzumab Ozogamicin alone showed modest benefit, but its integration with standard induction chemotherapy (such as the '7+3' regimen of cytarabine plus an anthracycline) markedly improved event-free survival and eradicated minimal residual disease (MRD).

Patient Selection Is Critical

Patient selection increasingly influences treatment success. The efficacy and safety of Gemtuzumab Ozogamicin vary with patients’ cytogenetic risk, underscoring the importance of biomarker-driven treatment strategies.

Toxicity Management Matters

Understanding unique ADC-associated toxicities is essential.

Multidisciplinary Care Matters

Successful treatment requires collaboration among physicians, nurses, pharmacists, and transplant specialists.

Key Takeaways

    • Gemtuzumab ozogamicin was the first antibody-drug conjugate approved for cancer treatment.
    • CD33 remains a validated therapeutic target in AML.
    • Fractionated dosing transformed the benefit-risk profile of gemtuzumab ozogamicin.
    • Clinical benefit has been demonstrated in newly diagnosed, relapsed, and pediatric AML populations.
    • The experience gained from gemtuzumab ozogamicin helped shape the development of modern ADC therapies.

Selected References

[1] Sievers EL, Larson RA, Stadtmauer EA, Estey E, Löwenberg B, Dombret H, Karanes C, Theobald M, Bennett JM, Sherman ML, Berger MS, Eten CB, Loken MR, van Dongen JJ, Bernstein ID, Appelbaum FR; Mylotarg Study Group. Efficacy and safety of gemtuzumab ozogamicin in patients with CD33-positive acute myeloid leukemia in first relapse. J Clin Oncol. 2001 Jul 1;19(13):3244-54. doi: 10.1200/JCO.2001.19.13.3244. PMID: 11432892. [2] Castaigne S, Pautas C, Terré C, Raffoux E, Bordessoule D, Bastie JN, Legrand O, Thomas X, Turlure P, Reman O, de Revel T, Gastaud L, de Gunzburg N, Contentin N, Henry E, Marolleau JP, Aljijakli A, Rousselot P, Fenaux P, Preudhomme C, Chevret S, Dombret H; Acute Leukemia French Association. Effect of gemtuzumab ozogamicin on survival of adult patients with de-novo acute myeloid leukaemia (ALFA-0701): a randomised, open-label, phase 3 study. Lancet. 2012 Apr 21;379(9825):1508-16. doi: 10.1016/S0140-6736(12)60485-1. Epub 2012 Apr 5. Erratum in: Lancet. 2018 Mar 3;391(10123):838. doi: 10.1016/S0140-6736(17)32105-0. PMID: 22482940. [3] Lambert J, Pautas C, Terré C, Raffoux E, Turlure P, Caillot D, Legrand O, Thomas X, Gardin C, Gogat-Marchant K, Rubin SD, Benner RJ, Bousset P, Preudhomme C, Chevret S, Dombret H, Castaigne S. Gemtuzumab ozogamicin for de novo acute myeloid leukemia: final efficacy and safety updates from the open-label, phase III ALFA-0701 trial. Haematologica. 2019 Jan;104(1):113-119. doi: 10.3324/haematol.2018.188888. Epub 2018 Aug 3. PMID: 30076173; PMCID: PMC6312010. [4] Hills RK, Castaigne S, Appelbaum FR, Delaunay J, Petersdorf S, Othus M, Estey EH, Dombret H, Chevret S, Ifrah N, Cahn JY, Récher C, Chilton L, Moorman AV, Burnett AK. Addition of gemtuzumab ozogamicin to induction chemotherapy in adult patients with acute myeloid leukaemia: a meta-analysis of individual patient data from randomised controlled trials. Lancet Oncol. 2014 Aug;15(9):986-96. doi: 10.1016/S1470-2045(14)70281-5. Epub 2014 Jul 6. PMID: 25008258; PMCID: PMC4137593. [5] Gamis AS, Alonzo TA, Meshinchi S, Sung L, Gerbing RB, Raimondi SC, Hirsch BA, Kahwash SB, Heerema-McKenney A, Winter L, Glick K, Davies SM, Byron P, Smith FO, Aplenc R. Gemtuzumab ozogamicin in children and adolescents with de novo acute myeloid leukemia improves event-free survival by reducing relapse risk: results from the randomized phase III Children’s Oncology Group trial AAML0531. J Clin Oncol. 2014 Sep 20;32(27):3021-32. doi: 10.1200/JCO.2014.55.3628. PMID: 25092781; PMCID: PMC4162498. [6] Appelbaum FR, Bernstein ID. Gemtuzumab ozogamicin for acute myeloid leukemia. Blood. 2017 Nov 30;130(22):2373-2376. doi: 10.1182/blood-2017-09-797712. Epub 2017 Oct 11. PMID: 29021230. [7] Fenwarth L, Podvin B, Vasseur L, Lebon D, Couillez G, Berthon C, Caulier A, Fournier E, Bories C, Carpentier B, Tricot S, Wattebled KJ, Roche-Lestienne C, Lestringant V, Hauspie C, Celli-Lebras K, Heiblig M, Dombret H, Itzykson R, Marolleau JP, Garçon L, Preudhomme C, Duployez N, Boyer T. Myeloid Neoplasms With Erythroid Predominance and Excess Blasts in Young Adults Exhibit Distinct Genetic Profiles. Eur J Haematol. 2025 Aug;115(2):185-192. doi: 10.1111/ejh.14435. Epub 2025 May 15. PMID: 40370262; PMCID: PMC12224554.


Learning Objectives


After completing this activity, participants should be able to:
  • Explain the biologic rationale for targeting CD33 in acute myeloid leukemia (AML).
  • Describe the mechanism of action of gemtuzumab ozogamicin.
  • Review the development, withdrawal, and reapproval of gemtuzumab ozogamicin.
  • Evaluate the pivotal clinical trial data supporting the use of gemtuzumab ozogamicin in AML.
  • Understand the evolution of dosing strategies that improved the benefit-risk profile of gemtuzumab ozogamicin.

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