Blood, 15 May 2001, Vol. 97, No. 10, pp. 3197-3204
NEOPLASIA
Targeting of the CD33-calicheamicin immunoconjugate Mylotarg
(CMA-676) in acute myeloid leukemia: in vivo and in vitro
saturation and internalization by leukemic and normal
myeloid cells
Vincent H. J. van der
Velden,
Jeroen G. te
Marvelde,
Patricia G. Hoogeveen,
Irwin D. Bernstein,
Adriaan B. Houtsmuller,
Mark S. Berger, and
Jacques J. M. van
Dongen
Antibody-targeted chemotherapy is a promising therapy in patients
with acute myeloid leukemia (AML). In a phase II study of Mylotarg
(CMA-676, gemtuzumab ozogamicin), which consists of a CD33 antibody
linked to calicheamicin, saturation and internalization by leukemic and
normal myeloid cells were analyzed in 122 patients with relapsed AML.
Peripheral blood samples were obtained just before and 3 and 6 hours
after the start of the first and second Mylotarg treatment cycles.
Within 3 to 6 hours after infusion, near complete saturation of CD33
antigenic sites by Mylotarg was reached for AML blasts, monocytes, and
granulocytes, whereas Mylotarg did not bind to lymphocytes. Saturation
levels prior to the start of the second Mylotarg treatment cycle were
significantly increased compared with background levels before the
start of the first cycle. This apparently was caused by remaining
circulating Mylotarg from the first treatment cycle (~2 weeks
earlier). On binding of Mylotarg to the CD33 antigen, Mylotarg was
rapidly internalized, as determined by the decrease in maximal surface
membrane Mylotarg binding. Internalization of Mylotarg was also
demonstrated in myeloid cells in vitro and was confirmed by confocal
laser microscopy. In vitro studies using pulse labeling with
Mylotarg showed a continuous renewed membrane expression of CD33
antigens, which can significantly increase the internalization process
and thereby the intracellular accumulation of the drug. Finally,
Mylotarg induced dose-dependent apoptosis in myeloid cells in vitro.
These data indicate that Mylotarg is rapidly and specifically targeted
to CD33+ cells, followed by internalization and subsequent
induction of cell death.
