Page 999 - Hematology_ Basic Principles and Practice ( PDFDrive )

882 Part VII Hematologic Malignancies

protein kinase C inhibitor GF109203X; the cyclosporin analog position of guanine. The most efficient means of protection from the
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PSC833; the TKI genistein; the gyrase-inhibiting antibiotic difloxa- cytotoxicity of adducts at the O position of guanine is rapid repair
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cin; and amiodarone VX-710. Amiodarone VX-710, a nonmacrocy- by the O -alkylguanine-DNA alkyltransferase (AGT or MGMT).
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clic ligand of the FK506-binding protein FKBP12 and a potent This protein serves as the stoichiometric acceptor protein for O -
modulator of PGP-mediated MDR, has been found to restore sensi- alkylguanine DNA monoadducts, transferring the alkyl group from
tivity of MRP-expressing HL60/ADR cells to the cytotoxic action of DNA to the active site of the protein, inactivating the protein, and
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doxorubicin, VP-16, and vincristine. Other investigational agents restoring DNA to normal. However, the N G–N C DNA cross-link
include danusertib, a potent pan-aurora and ABL kinase inhibitor that follows chloroethylation is not a substrate for AGT. There is a
being developed for CML. The nonsteroidal antiinflammatory drug striking correlation between drug resistance and alkyltransferase
indomethacin has also been shown to significantly increase the sen- activity. Of interest, high levels of AGT are found in many leukemias,
sitivity of HL60/ADR cells to doxorubicin and vincristine, and may but low AGT is observed in normal human CD34 cells, perhaps
be a specific inhibitor of MRP. 19 explaining why nitrosoureas are not used in leukemia management
and why nitrosoureas are effective myeloablative agents used in high-
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Multidrug-Resistance Protein Expression in dose chemotherapy regimens. A novel inhibitor of AGT, O -
benzylguanine (BG) has been used to sensitize human tumors to
Hematologic Malignancies BCNU. Studies indicate activity of the combination of BG and
BCNU in myeloma and in cutaneous lymphomas. Therapeutic
The expression of MRP mRNA or the level of MRP protein has benefit from the methylating and alkylating agent cloretazine appears
been assessed in 148 patients with hematopoietic malignancies. MRP to be related to lower levels of AGT. Likewise, temozolomide has
mRNA expression was found to be significantly increased in 84% of some therapeutic efficacy in leukemia in a manner inversely related
patients with CLL and in 30% of those with AML. The vast majority to AGT levels in the tumor cells. A study in five pediatric patients
of patients with ALL, CML, MM, hairy cell leukemia, and NHL were pointed out that response to temozolomide was greater in those with
found to have low levels of MRP mRNA expression. MRP protein MGMT promoter methylation and no evidence of MMR. This has
was assayed using the monoclonal antibody MRPr1, and the results been tested in a prospective phase II trial of older patients with leu-
were generally similar with increased expression in most patients with kemia. Temozolomide was given in a dosing schedule dependent on
CLL. A study of 40 patients with refractory lymphoma and 16 with MGMT promoter methylation status, with a shorter course of
newly diagnosed lymphoma suggests a limited role for MRP mRNA therapy in those with low MGMT expression in the leukemic cells.
expression in drug resistance in NHL because 15 paired samples The ORR in elderly leukemia patients who would otherwise not be
in the refractory group showed no difference in MRP expression candidates for treatment was about 40%, with a median duration of
pre- and post-EPOCH treatment. In addition, the untreated patients about 29–35 weeks.
had MRP mRNA levels that were no different from the pre- or post-
EPOCH patient levels. A study of 49 patients with AML and 29 with Mismatch Repair
ALL demonstrated significantly higher expression of MRP in ALL (p The spectrum of drug resistance and sensitivity to methylating agents
< .007) and in secondary AML (p < .016), but not in de novo AML. does not end with AGT. Evidence suggests that methylating agent-
Combining overexpression of ABC G2 and FLT3-ITD, an Italian induced cell death involves an aborted effort at MMR. Karran and
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group identified that in adult AML patients, overexpression of ABC others, in mammalian systems, have shown that the replicative DNA
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G2 was detected in 83 (50%) and FLT3-ITD in 47 (28%) patients. polymerase and the repair polymerase pauses at O -mG and prefer-
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Although the response rate was not affected, the duration of remission entially inserts a thymine (T) at the site. The O -mG:T base pair is
was much shorter when these proteins were overexpressed. In a small recognized by the MMR system, which initiates (see later). In human
study of 14 patients with relapsed AML, relapse was associated with cells, the MMR complex consists of at least six proteins involved in
a twofold increase in blast MRP mRNA relative to 29 patients with the recognition and repair of mismatch lesions: HMLH1, hMSH2,
newly diagnosed AML (p < .01). Paired blast samples (obtained at hMSH3, hPMS1, hPMS2, and GTBP (GT-binding protein, also
diagnosis and at relapse) from 13 AML and four ALL patients showed called MSH6), all of which appear to be homologs of MMR proteins
a twofold increase in 80% of the patients at relapse, suggesting that found in Escherichia coli and in yeast. After binding recognition,
the expression of the MRP transporter at relapse may be involved in an endonuclease removes a patch of approximately 100–1000 bp
drug resistance. Because purine nucleoside antimetabolites may be containing the T mismatch, DNA polymerase-δ or -γ fills in the
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exported by ABC G2, as noted earlier, it is of note that clofarabine patch, with reinsertion of a T opposite the O -mG, and a DNA
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is also a substrate, but its function is mediated inversely by levels of ligase closes the strand break. Because the O -mG:T is reformed,
deoxycytidine kinase. Also of note is the finding that drug treatment cytotoxicity ensues as a result of repetitive efforts at DNA repair
can demethylate and thus activate the ABC G2 exporter, thereby and induction of chromosomal breakage, rearrangements, energy
increasing its protective tumor impact. depletion, and apoptosis. Drug resistance based on mutation or loss
The larger family of transporters, the ABC group that includes of expression of an MMR protein, owing to mutation within one of
ABC G2 and ABC B1, are overexpressed in leukemic cells and in the gene coding regions, or promoter methylation leading to loss of
many malignant stem cells. In fact, in addition to CD44, high levels gene expression, has been noted in solid tumors and in leukemias
of ABC transporters assist in the characterization and isolation of the and lymphomas. The mutator phenotype was originally described in
transplantable subpopulation of malignant stem cells. Brendel and cells with acquired resistance to methylnitrosourea, methylmethane-
coworkers found that both imatinib and nilotinib used to block the sulfonate, or N-methyl-N-nitroso-N-nitrosoguanidine, which were
ABL kinase in CML were effectively blocking the ABC transporter tolerant to G→A point mutations according to the inability to repair
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in CML cells. This suggested both that these agents might function O -mG and are cross-resistant to 6-TG used in childhood leukemia
in part by blocking expression of an important leukemic stem cell maintenance regimens, which form the 6-TG:T mismatch.
protein, and that they would sensitize CML cells to other agents
transported by the ABC system. Major Molecular Response Mutations and Methylating
Agent Resistance
DNA Repair Pathway Mechanisms of Drug MMR defects in humans were initially described in hereditary
nonpolyposis colon cancer, which comprises approximately 15%
Resistance of all colon cancer, lymphomas, and relapsing acute leukemias.
The genetic defect results in a high rate of spontaneous mutations
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O -Alkylguanine-DNA Alkyltransferase within microsatellite DNA, resulting in the RER phenotype arising
As noted previously, the nitrosoureas and methylating agents are as the expansion or contraction of mono-, di-, or tri-nucleotide
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cytotoxic largely because of formation of DNA adducts at the O repeats within the microsatellites. Tumor cells defective in MMR are