Multidrug Drug Resistance in Pediatric Sarcomas

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

Induction of drug resistance in human rhabdomyosarcoma cell lines is associated with increased maturation: possible explanation for differentiation in recurrences?

In rhabdomyosarcoma (RMS) of childhood and adolescence very little is known about interactions of cytotoxic drugs and tumor cells. In recurrent RMS the tumor cells are often more mature than in the primary tumor. The biological properties of these cells are still a subject of controversy. We investigated two human (RD2 and TE 671) cell lines by cultivating them with doxorubicin, cisplatinum, and etoposide. Degree of differentiation and proliferation rate were estimated morphologically and by means of immunohistochemistry and a monolayer proliferation assay. Both morphological and immunohistochemical maturation was measurable in most resistant cell lines. An increase in myosin expression was most marked in the etoposide- and doxorubicin-resistant RD cell lines. The proliferation rate was decreased in almost all resistant cell lines. Nevertheless, the resistant cell lines tolerated high-dose levels of cytotoxic drugs at a higher proliferation rate than parental cell lines cultivated under similar conditions. The maturation seen in some recurrent tumors of RMS can be simulated in vitro by cultivating cell lines with cytotoxic drugs at sublethal doses. Interestingly, the resistance-associated induced maturation was not accompanied by p170 expression. After comparing these in vitro results with the maturation seen in RMS specimens after chemotherapy, we conclude that chemotherapy-induced differentiation in vivo might be a morphological sign of chemoresistance.

The 16p11 breakpoint in myxoid liposarcomas might affect the expression of the LRP gene on 16p11.2 encoding the multidrug resistance associated major vault protein

BACKGROUND

Chromosome breakage could influence the expression of genes. This has been noticed in specific cases of acute myeloid leukaemia, where the 16p13 breakpoint affects the expression of the multidrug resistance related protein (MRP). Myxoid liposarcomas (LPS) are characterized by the t(12; 16)(q13; p11), which leads to the formation of a FUS-CHOP fusion transcript. This study investigates the relationship between the cytogenetically detected breakpoint 16p11 in myxoid LPS, the presence of the FUS-CHOP fusion transcript in nonmyxoid LPS and the expression of the lung resistance major vault protein (LRP) gene on 16p11.2.

MATERIALS AND METHODS

Of 16 cases with a diagnosis of a (possible) liposarcoma with an abnormal karyotype, fresh frozen tumour material was available for immunohistological detection of LRP. Cases without a cytogenetically detected t(12; 16)(q13; p11), were analyzed for the presence of a FUS-CHOP fusion transcript by RT-PCR.

RESULTS

In all 9 myxoid LPS a t(12; 16)(q13; p11) was found and LRP expression was absent or low. In none of the remaining 7 cases was a FUS-CHOP fusion transcript found, and four tumours were LRP positive (P = 0. 02). LRP expression in myxoid LPS (mean: 1.3%) was lower (P = 0.07) than in the nonmyxoid tumours (mean: 35.7%).

CONCLUSIONS

These observations indicate a relation between the t(12; 16)(q13; p11), leading to a FUS-CHOP fusion transcript in myxoid LPS, and the low or absent expression of the LRP-gene located on 16p11.2.

Assessment of systemic toxicity in children receiving chemotherapy with cyclosporine for sarcoma

BACKGROUND

Overexpression of P-glycoprotein in malignant tumors has been associated with poor responses to chemotherapy. It appears biologically plausible that addition of the P-glycoprotein inhibitor cyclosporine (CsA) to standard chemotherapy may improve the outcome. The protective functions of P-glycoprotein in healthy tissues, however, have not been fully elucidated. Addition of CsA may lead to increased systemic chemotherapy toxicity, so we compared the rate and severity of chemotherapy-associated systemic toxicity in the presence and absence of CsA.

PROCEDURE

Standard chemotherapy consisted of etoposide/ifosfamide (VP16/IFOS) cycles, alternating with vincristine/dactinomycin/cyclophosphamide (VAC) cycles. CsA was given at a median dose of 20 mg/kg with unaltered doses of the antineoplastic drugs. The analysis of toxicity was performed by comparing clinically significant toxicity events recorded during and after chemotherapy cycles with and without CsA.

RESULTS

Toxicity-related hospital admissions occurred after 93% of VAC cycles with CsA compared to 40% of the cycles without CsA (P < 0. 0001); 29% of VP16/IFOS cycles with CsA led to admissions vs. 12% with non-CsA cycles (P = 0.04). Infections or fever and neutropenia were the main reasons for these admissions. Thirty-seven percent of the VAC cycles with CsA were complicated by culture-proved sepsis, which did not occur in cycles without CsA (P < 0.0001). Requirements for blood and platelet transfusions were greatly increased after VAC cycles with CsA compared to VAC cycles without CsA.

CONCLUSIONS

The chemosensitizer CsA increases the systemic toxicity of VAC chemotherapy in patients with sarcomas. Future trials of chemotherapy with chemosensitizers will have to take into account a potential increase in systemic toxicity. Careful monitoring of chemotherapy-related toxicity becomes mandatory in such studies.

Multidrug resistance phenotype in high grade soft tissue sarcoma: correlation of P-glycoprotein immunohistochemistry with pathologic response to chemotherapy

BACKGROUND

P-glycoprotein-mediated drug efflux has been implicated as an important mechanism of multidrug resistance (MDR) in cancer. Its role in chemotherapy resistance in soft tissue sarcoma is unclear.

METHODS

Tumor specimens prior to and following neoadjuvant chemotherapy from 29 cases of high grade soft tissue sarcoma were analyzed with 2 monoclonal antibodies (C494 and JSB-1) that recognize different epitopes of P-glycoprotein. Staining intensity was graded 0 = negative, 1 = equivocal, 2 = moderate, 3 = strong. Only cases with Grade 2 or 3 staining intensity with both antibodies were considered MDR positive. The resection specimens were evaluated for tumor necrosis postchemotherapy. Pathologic response was graded as good for <15%, moderate for 15-50%, or poor for >50% posttreatment tumor viability.

RESULTS

Of the 29 pretreatment specimens, 10 (34%) were MDR positive and 19 (66%) were MDR negative. Pathologic response to treatment was characterized as good in 6, moderate in 7, and poor in 16 patients. Of the MDR positive biopsies, 9 (90%) had poor response, compared with 7 (36%) in the MDR negative biopsy group (P = 0.0078). None of the cases with MDR positive biopsies had a good response, compared with 6 cases in which biopsies were MDR negative (32%) (P = 0.057). Only one MDR negative case became MDR positive posttreatment.

CONCLUSIONS

Expression of MDR phenotype is found in approximately one-third of high grade soft tissue sarcomas. These preliminary data show a significant correlation between MDR phenotype and poor pathologic response to chemotherapy, and suggest that MDR induction by chemotherapy in soft tissue sarcoma is an uncommon event.

A single chain Fv fragment of P-glycoprotein-specific monoclonal antibody C219. Design, expression, and crystal structure at 2.4 A resolution

A construct encoding a single chain variable fragment of the anti-P-glycoprotein monoclonal antibody C219 was made by combining the coding sequences for the heavy and light chain variable domains with a sequence encoding the flexible linker (GGGGS)3, an OmpA signal sequence, a c-myc identification tag, and a five-histidine purification tag. The construct was expressed in Escherichia coli and purified from the periplasmic fraction using a nickel chelate column and ion exchange chromatography. Three-step Western blot analysis showed that the construct retains binding affinity for P-glycoprotein. Crystals of 1.0 x 0.2 x 0.2 mm were grown in 100 mM citrate, pH 4.5, 21% polyethylene glycol 6000 in the presence of low concentrations of subtilisin, resulting in proteolytic removal of the linker and purification tags. The structure was solved to a resolution of 2.4 A with an R factor of 20.6, an Rfree of 28.5, and good stereochemistry. This result could lead to a clinically useful product based on antibody C219 for the diagnosis of P-glycoprotein-mediated multidrug resistance. The molecule will also be useful in biophysical studies of functional domains of P-glycoprotein, as well as studies of the intact molecule.