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Departments of
Pharmaceutical Sciences (A.J.G., J.D.S., E.M., Y.Y.,
T.W.S., M.V.R., W.E.E.) and
Hematology-Oncology (C.-H.P.), St. Jude
Children's Research Hospital, and the
Center for Pediatric
Pharmacokinetics and Therapeutics (M.V.R., W.E.E., C.-H.P., J.D.S),
Colleges of Pharmacy and Medicine, University of Tennessee, Memphis,
Tennessee 38101, and
Division of Pediatric Hematology-Oncology, Medical
University of South Carolina, Charleston, South Carolina (J.C.B.)
Cellular accumulation of methotrexate polyglutamates (MTXPGs) is
recognized as an important determinant of the cytotoxicity and
selectivity of methotrexate in acute lymphoblastic leukemia (ALL). We
identified a significantly lower cellular accumulation of MTXPGs in
T-lineage versus B-lineage lymphoblasts in children with ALL, which is
consistent with the worse prognosis of T-lineage ALL when treated with
conventional antimetabolite-based therapy. Maximum MTXPG accumulation
in leukemic blasts in vivo was 3-fold greater in
lymphoblasts of children with B-lineage ALL (129 children) compared
with those with T-lineage ALL (20 children) (p < 0.01) and was characterized by a saturable
(Emax) model in both groups. The human
leukemia cell lines NALM6 (B-lineage) and CCRF/CEM (T-lineage) were
used to assess potential mechanisms for these lineage differences in
MTX accumulation, revealing i) greater total and long-chain MTXPG
accumulation in NALM6 over a wide range of methotrexate concentrations
(0.2-100 µM), ii) saturation of MTXPG accumulation in
both cell lines, with a higher maximum
(Emax) in NALM6, iii) 3-fold higher
constitutive FPGS mRNA expression and enzyme activity in NALM6 cells,
iv) 2-fold lower levels of DHFR mRNA and protein in NALM6 cells, and v)
4-6 fold lower extracellular MTX concentration and 2-fold lower
intracellular MTXPG concentration to produce equivalent cytotoxicity
(LC50) in NALM6 versus CEM. There was a significant
relationship between FPGS mRNA and enzyme activity in lymphoblasts from
children with newly diagnosed ALL, and blast FPGS mRNA and activity
increased after methotrexate treatment. These data indicate higher FPGS
and lower DHFR levels as potential mechanisms contributing to greater
MTXPG accumulation and cytotoxicity in B-lineage lymphoblasts.
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