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.

MYLOTARG® (gemtuzumab ozogamicin) 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, MYLOTARG 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

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 involved in regulation of 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, allowing preservation of 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 on 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 consist of three essential components:

1. Monoclonal antibody
2. Linker technology
3. Cytotoxic payload

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 upon the optimization of 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

MYLOTARG 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 CD33 on AML blasts.

Binding initiates rapid internalization of the antibody-antigen complex.

Linker Technology

The antibody is conjugated through a hydrolyzable linker to a derivative of calicheamicin.

After internalization, lysosomal degradation and intracellular processing facilitate release of the cytotoxic payload.

Although 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 MYLOTARG

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, MYLOTARG 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 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 MYLOTARG from the market in 2010.

For many observers, the withdrawal raised broader questions regarding the future viability of ADC technology.

The Rebirth of MYLOTARG

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 pivotal ALFA-0701 study enrolled patients aged 50 to 70 years with newly diagnosed AML.

Patients received standard induction chemotherapy consisting of daunorubicin and cytarabine with or without fractionated gemtuzumab ozogamicin.

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.

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 MYLOTARG as a treatment option beyond traditional intensive chemotherapy settings.

AAML0531: Pediatric AML

The Children's Oncology Group AAML0531 trial extended 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 MYLOTARG Development

Several important principles emerged from the MYLOTARG experience:

Dose Matters

Optimization of dosing schedules can dramatically alter an ADC's therapeutic index.

Target Selection Matters

CD33 remains one of the most validated targets in AML.

Biomarkers Matter

Patient selection increasingly influences treatment success.

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

• MYLOTARG 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 MYLOTARG helped shape the development of modern ADC therapies.

Selected References

After completing this activity, participants should be able to:

1. Explain the biologic rationale for targeting CD33 in acute myeloid leukemia (AML).
2. Describe the mechanism of action of gemtuzumab ozogamicin.
3. Review the development, withdrawal, and reapproval of MYLOTARG.
4. Evaluate the pivotal clinical trial data supporting the use of MYLOTARG in AML.
5. Understand the evolution of dosing strategies that improved the benefit-risk profile of gemtuzumab ozogamicin.