Mylotarg: Regulatory History, Approval Timeline, and Critical Safety Warnings

Table of Contents#

1. What Is Mylotarg (Gemtuzumab Ozogamicin)?
2. Full Regulatory History of Mylotarg 2.1 2000 Initial Accelerated FDA Approval 2.2 2010 Voluntary Market Withdrawal 2.3 2017 FDA Reapproval: Dosing Update and Expanded Indications 2.4 Global Regulatory Approvals
3. Critical Safety Warnings for Mylotarg 3.1 FDA Boxed Warnings (Highest Severity) 3.2 Common Manageable Adverse Events 3.3 Special Population Precautions
4. Key Takeaways for Patients and Clinicians
5. References

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What Is Mylotarg (Gemtuzumab Ozogamicin)?#

Mylotarg is a CD33-directed antibody-drug conjugate (ADC), a type of targeted therapy that combines a monoclonal antibody with a chemotherapy payload. The antibody binds to the CD33 protein, which is expressed on 90% of AML cancerous blast cells, and delivers the chemotherapy drug calicheamicin directly to the cancer cells to minimize damage to healthy tissue.

As of 2026, Mylotarg is approved for the following use cases:

1. Newly diagnosed CD33-positive AML in adult patients and pediatric patients 1 month and older
2. Relapsed or refractory CD33-positive AML in adult patients and pediatric patients 2 years and older

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Full Regulatory History of Mylotarg#

Mylotarg’s regulatory path is a widely cited case study for the value of post-approval confirmatory trials for drugs granted accelerated access.

2000 Initial Accelerated FDA Approval#

In May 2000, the FDA granted Mylotarg accelerated approval for the treatment of CD33-positive relapsed AML in patients aged 60 and older who were not candidates for intensive chemotherapy. Accelerated approval is a pathway for drugs addressing serious unmet medical needs, where approval is granted based on surrogate endpoints (biomarkers reasonably likely to predict clinical benefit) rather than proven overall survival.

The approval was based on data from 3 single-arm trials including 142 patients, which showed a 26% overall response rate, with a median duration of response of 7 months. The approved dosing schedule at the time was 9 mg/m² administered as a 2-hour infusion every 2 weeks, for a total of 2 doses.

2010 Voluntary Market Withdrawal#

In June 2010, Pfizer voluntarily withdrew Mylotarg from the US market after the confirmatory phase 3 SWOG S0106 trial failed to meet its primary endpoint: patients treated with Mylotarg plus standard chemotherapy had no improvement in overall survival compared to patients treated with chemotherapy alone.

Worse, the trial found a significantly higher rate of fatal adverse events in the Mylotarg arm (5.5% vs 1.4% in the control arm), driven primarily by hemorrhagic events (including central nervous system hemorrhage) and infections. Notably, the trial used a lower daunorubicin dose (45 mg/m²) in the Mylotarg arm compared to the control arm (60 mg/m²), which is now thought to have negatively influenced outcomes independent of Mylotarg.

2017 FDA Reapproval: Dosing Update and Expanded Indications#

Following the 2010 withdrawal, researchers investigated whether a revised, fractionated dosing schedule could reduce toxicity while preserving efficacy. Instead of a high 9 mg/m² dose given twice, trials tested a lower 3 mg/m² dose administered on days 1, 4, and 7 of induction therapy, with optional 3 mg/m² doses during consolidation cycles.

The phase 3 ALFA-0701 trial of 278 newly diagnosed AML patients aged 50 to 70 found that patients treated with the fractionated Mylotarg schedule plus chemotherapy had a 40% reduction in risk of disease progression or death, with event-free survival of 17.3 months vs 9.5 months in the chemotherapy-only arm. Fatal toxicity rates were comparable between the two arms. Additional trial data in pediatric relapsed/refractory AML showed a 48% overall response rate.

In September 2017, the FDA reapproved Mylotarg with the updated fractionated dosing schedule, and expanded indications to include newly diagnosed CD33-positive AML in adults and pediatric patients, as well as relapsed/refractory disease.

Global Regulatory Approvals#

• June 2020: FDA extended the newly diagnosed CD33-positive AML indication to include pediatric patients 1 month and older, based on data from the AAML0531 trial
• 2018: European Medicines Agency (EMA) approved Mylotarg for the same indications as the FDA
• 2019: Japan’s Pharmaceuticals and Medical Devices Agency (PMDA) approved Mylotarg
• 2018: UK’s National Institute for Health and Care Excellence (NICE) recommended Mylotarg for routine NHS use for eligible AML patients (TA545)

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Critical Safety Warnings for Mylotarg#

Mylotarg carries strict safety guidance from all global regulatory bodies, including mandatory monitoring requirements for clinicians.

FDA Boxed Warnings (Highest Severity)#

Boxed warnings are the FDA’s strongest warning for drugs with significant risks of life-threatening adverse events:

1. Hepatotoxicity, including fatal VOD/SOS: Risk of VOD is highest in patients who receive Mylotarg before or after hematopoietic stem cell transplant (HSCT), patients with pre-existing liver impairment, and patients treated with higher, non-fractionated doses. Clinicians must monitor liver function tests before each dose, and monitor patients for signs of VOD (rapid weight gain, right upper quadrant pain, jaundice, ascites) for at least 1 month after treatment.
2. Severe infusion reactions, including anaphylaxis: Reactions typically occur within 24 hours of infusion, and may include fever, chills, hypotension, wheezing, or swelling of the face/throat. Patients must receive pre-medication with antihistamines, acetaminophen, and corticosteroids before each infusion, and emergency resuscitation equipment must be available during administration.
3. Severe myelosuppression leading to fatal infection or bleeding: Mylotarg causes prolonged neutropenia (low white blood cell count) and thrombocytopenia (low platelet count) in most patients. Clinicians must monitor complete blood counts weekly during treatment, and provide prophylactic antibiotics and white blood cell growth factors as needed to reduce infection risk.

Common Manageable Adverse Events#

Most non-life-threatening side effects of Mylotarg are manageable with supportive care, and include:

• Nausea, vomiting, and diarrhea (occur in 30-40% of patients)
• Fatigue and fever
• Mucositis (mouth sores)
• Mild rash
• Headache

Special Population Precautions#

• Pregnancy and breastfeeding: Mylotarg can cause severe fetal harm. Patients of reproductive age should use effective contraception during treatment and for 6 months after the last dose. Breastfeeding is not recommended during treatment and for 1 month after the last dose.
• Pediatric patients: Safety is established for patients 1 month and older, with a similar adverse event profile to adults, though VOD risk is elevated post-HSCT.
• Renal/hepatic impairment: No dose adjustment is needed for mild to moderate renal impairment, or mild hepatic impairment. Patients with moderate to severe hepatic impairment should be monitored closely for hepatotoxicity, with dose holds or adjustments as needed.

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Key Takeaways for Patients and Clinicians#

1. Mylotarg’s 2010 withdrawal and 2017 reapproval highlight the importance of post-approval confirmatory trials for drugs granted accelerated access, and the value of optimized dosing schedules to reduce toxicity.
2. Eligible patients should discuss the benefits and risks of Mylotarg with their oncology team, and immediately report any signs of infusion reaction, liver issues, or infection during treatment.
3. Clinicians must follow the approved fractionated dosing schedule, and adhere to mandatory monitoring requirements for hepatotoxicity, myelosuppression, and infusion reactions.

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References#

1. U.S. Food and Drug Administration. (2021). Mylotarg (gemtuzumab ozogamicin) Prescribing Information. Retrieved from https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=32fd2bb2-1cfa-4250-feb8-d7956c794e05
2. U.S. Food and Drug Administration. (2017, September 1). FDA approves gemtuzumab ozogamicin for CD33-positive AML. Retrieved from https://www.fda.gov/drugs/drug-approvals-and-databases/fda-approves-gemtuzumab-ozogamicin-cd33-positive-aml-pediatric-patients
3. Petersdorf, S. H., et al. (2013). Gemtuzumab ozogamicin for acute myeloid leukemia in older patients. New England Journal of Medicine, 369(4), 323-332.
4. Castaigne, S., et al. (2012). Gemtuzumab ozogamicin as first-line treatment for older patients with acute myeloid leukaemia (ALFA-0701): a randomised, open-label, phase 3 trial. Lancet Oncology, 13(10), 1021-1031.
5. European Medicines Agency. (2018). Mylotarg: EPAR - Summary of Product Characteristics. Retrieved from https://www.ema.europa.eu/en/medicines/human/EPAR/mylotarg
6. National Institute for Health and Care Excellence. (2018). Gemtuzumab ozogamicin for untreated acute myeloid leukaemia in adults. Retrieved from https://www.nice.org.uk/guidance/ta545
7. National Cancer Institute. (2026). SEER Cancer Stat Facts: Acute Myeloid Leukemia. Retrieved from https://seer.cancer.gov/statfacts/html/amyl.html
8. U.S. Food and Drug Administration. (2020, June 16). FDA approves gemtuzumab ozogamicin for CD33-positive AML in pediatric patients. Retrieved from https://www.fda.gov/drugs/drug-approvals-and-databases/fda-approves-gemtuzumab-ozogamicin-cd33-positive-aml-pediatric-patients