To our knowledge, this is the first report of an inducible in vivo
model for multidrug resistance in human high-risk ALL with chromosomal translocation t(4:11). MDR is often studied by generating MDR-resistant sublines from cultured cells and either studying these sublines in vitro
or injecting the resistant clonal cells into immunodeficient mice. Resistant sublines have been utilized to study MDR in many types of cancers in vivo
, including T-cell leukemia, Burkitt’s lymphoma, and promyelocytic leukemia [16
]. The SEM cell line used for this study was established in culture from a relapsed patient with ALL containing the translocation t(4;11) [14
], and treatment of the patient may have induced an initial MDR state prior to removal of the cells and establishment of the cell line. However, without selective pressure, these cells lost PgP expression and were sensitive to vincristine in vitro
. Upon engraftment into NOD/SCID mice and subsequent treatment with vincristine, SEM cells increased expression of the PgP protein and likely other MDR transport proteins, and resulted in reversion to a drug resistant phenotype. A delay in leukemia cell growth in the mice and increased survival were observed after vincristine treatment compared to control, but all of the mice succumbed to disease. This model represents an inducible system that is superior to injecting already resistant sublines and may be used to evaluate step-wise in vivo mechanisms involved in the development of or reversion to a multidrug resistant state in these cells.
The greatest obstacle for successful chemotherapy against leukemia is the development of cells that are resistant to the therapeutic agents. Olson et al. [20
] recently showed that prognosis of pediatric ALL patients was not significantly dependent upon overexpression of the MDR efflux pumps PgP, MRP or LRP in newly diagnosed patients. PgP was upregulated or overexpressed in only about 2% (5 of 295) of the patient samples taken at the time of diagnosis and the small number did not allow statistical analysis of a correlation between PgP expression and survival. Lack of correlation of PgP, MRP-1, and LRP expression and therapeutic success against pediatric ALL at diagnosis was also shown by others [21
]. However, Styczynski et al. [24
] showed patients with relapsed ALL had a strong trend toward adverse impact of PgP, MRP1, and LRP expression on prognosis. These data suggest that these efflux and transport systems play an important role in the greater lack of success of conventional chemotherapeutics used for patients with relapsed ALL. It should be noted that the studies referenced above did not specifically differentiate the subtypes of ALL in the analysis groups, so the actual impact of PgP expression on the prognosis and overall survival in patients with high-risk t(4;11) ALL is not clear, whether at diagnosis or after relapse.
Vincristine is a neurotoxic therapeutic agent that can induce cumulative peripheral neuropathy and epileptic seizures and is usually administered once per week to the human patient [25
]. We did not observe overt signs of intolerance to the multiple dosing of vincristine (3× per week), such as weakness, palsy, or changes in behavior when compared to the condition of the PBS-treated mice. However, the mice in both groups showed similar changes in weight before and after engraftment of the leukemia cells. By day 25, the average percent of CD19+ SEM cells was 13.96 ± 1.75 for PBS-treated mice compared to 3.86 ± 1.0 for vincristine-treated mice. Since the leukemia burden was significantly higher in the PBS-treated mice which would make these mice more ill, the lack of a difference in weight loss between the two groups may reflect some toxicity induced by the vincristine treatment.
Palucka et al. [26
] showed that leukemic cells from patients at relapse engrafted more rapidly in SCID mice than cells from newly diagnosed patients. SEM cells engrafted in the NOD/SCID mice approximately 2 weeks after injection into the tail vein, whereas Leim et al. [10
] showed that biopsy material took an average of 43 ± 5 days for engraftment to occur. The rapid engraftment of SEM cells and reversion to chemotherapy resistance after vincristine treatment provides a useful model for the study of MDR in high-risk leukemia without the need to obtain patient biopsy material. This model is also relatively uncomplicated and provides an important means for developing novel treatment strategies that overcome MDR mechanisms in this often fatal disease.