CD33 expression (CD33-positive)

aka CD33-positive, CD33+, CD33 expression, CD33 positive AML

CD33 is a myeloid antigen on the blasts of about 90 percent of acute myeloid leukaemias. Gemtuzumab ozogamicin's label requires CD33-positive disease.

CD33 is measured by flow cytometry on marrow or blood blasts. Gemtuzumab ozogamicin (Mylotarg) is a CD33-directed antibody-drug conjugate labelled for newly diagnosed CD33-positive AML with chemotherapy in adults and children from one month, and relapsed or refractory CD33-positive AML; the label does not set a percentage, though trials used 20 percent or more of blasts and benefit is greatest in favourable-risk cytogenetics. CD33 is also the target of bispecifics and CAR-T cells in trials.

Part ofCD33· CD33 All readouts

CD33: what it is and how drugs act on it · animated schematic, not to scale

Target · the protein and the cell it sits on
Drug · antibody, small molecule, cell or radioligand
Effect · signal, damage or kill

In plain words · CD33 is a myeloid surface marker on the blasts of 85 to 90% of acute myeloid leukaemias and on normal myeloid cells, so drugs against it also hit healthy marrow. It is the target of gemtuzumab ozogamicin, the first ADC ever approved (2000), withdrawn in 2010 and re-approved in 2017 at a lower fractionated dose.

IHC score 1 approval threshold

What it means for a patient

A CD33-positive result, present in most acute myeloid leukaemias, means gemtuzumab ozogamicin can be added to your first chemotherapy or used if the disease returns; it helps most when the leukaemia's chromosomes are in the favourable group, so both results are read together.

Written only from the label or guideline text cited on this page. Not medical advice; your own report and the reading your team gives it come first.

Scoring rule

CD33 detected on leukaemia blasts by flow cytometry; the label states CD33-positive without a percentage.

“MYLOTARG is a CD33-directed antibody and cytotoxic drug conjugate indicated for: • treatment of newly-diagnosed CD33-positive acute myeloid leukemia (AML) in adults and pediatric patients 1 month and older”
Mylotarg prescribing information

Thresholds used in approvals

Threshold	Drug	Cancer	Regulator	Source
CD33-positive	Gemtuzumab ozogamicin	Acute myeloid leukaemia	FDA	label

Drugs whose label names it

Gemtuzumab ozogamicin

Trials that name this readout

Matched on the name and aliases of the readout in the title, setting and summary of each trial; a match is a mention, not proof the readout was an entry criterion.