Multidrug resistance phenotype in leukaemic cells from patients with acute myelocytic leukaemia can be detected with 99Tc(m)-MIBI

MDR1, MRP1 and LRP expression in patients with untreated acute leukaemia: Correlation with 99mTc-MIBI bone marrow scintigraphy

BACKGROUND

Chemotherapy failure linked to multidrug-resistance (MDR) plays an important role in many cancer types, including leukaemia. It is believed that overexpression of some of membrane or intracellular proteins confers the MDR phenotype to cancer cells. (99m)Tc-sestamibi (MIBI) is a transport substrate for the Pgp pump. We assessed the bone marrow uptake of (99m)Tc-MIBI and its correlation with messenger RNA (mRNA) levels of MDR1, multidrug-resistance associated protein-1 (MRP1) and lung resistance protein (LRP) in acute leukaemia.

METHODS

A total of 26 patients (age range 17-72 years; mean age 51.88+/-2.52 years) with newly diagnosed acute leukaemia were included in the study. The expression of MDR1, MRP1 and LRP on mRNA levels were assessed by quantitative RT-PCR in the blast cells. The MIBI uptake in the bone marrow was evaluated using a quantitative scoring system with determination of the tumour-to-background ratios for the bone marrow areas. The correlation between the quantitative RT-PCR results and MIBI uptake was analysed by using Spearman's rank correlation coefficients with two-tailed test of significance.

RESULTS

There was an inverse relationship between (99m)Tc-MIBI uptake of bone marrow and both mRNA levels of MDR1 and MRP1 (P=0.000, r= -0.733 and P=0.001, r= -0.610, respectively). No correlation was found between MIBI uptake and mRNA levels of LRP.

CONCLUSION

Increased expression of MDR1 and MRP1 correlates with a low accumulation of (99m)Tc-MIBI in bone marrow areas in patients with acute leukaemia. (99m)Tc-MIBI bone marrow scintigraphy can identify the MDR1 and MRP1 phenotype, but not LRP, in patients with acute leukaemia.

Assessment of the P-glycoprotein expression by 99mTc-MIBI bone marrow imaging in patients with untreated leukaemia

The ability of cancer cells to become simultaneously resistant to different drugs is a significant impediment to successful chemotherapy. 99mTc-MIBI has been reported to be a transport substrate for P-glycoprotein (Pgp). The aim of the study was to ascertain the relationship between the degree of 99mTc-MIBI uptake and the level of Pgp expression in patients with newly diagnosed leukaemia. A total of 26 patients (12 female and 14 male; mean age 46.8+/-3.7 years) with newly diagnosed leukaemia were included in the study. None of the patients had been previously treated with chemotherapy. Images were obtained 20 min post-injection of 740 MBq 99mTc-MIBI. Whole-body and planar spot images of the pelvis and thorax were acquired. The uptake of the MIBI in the bone marrow was evaluated using a qualitative and also a quantitative scoring system with determination of the tumour-to-background (T/B) ratios. Flow cytometry was performed for determining the Pgp expression of the blast cells in the bone marrow aspiration samples. There was a statistically significant inverse relationship between the Pgp level in numeric values and both mean qualitative (P<0.001; r=-0.665) and quantitative (P=0.001; r=-0.606) results of 99mTc-MIBI imaging. Both the mean qualitative score and the T/B ratios were higher in patients who were Pgp negative than in those who were Pgp positive (P<0.001 and P<0.001, respectively). These data indicate that an increased level of Pgp expression is correlated with a low accumulation of 99mTc-MIBI in bone marrow of patients with leukaemia. 99mTc-MIBI bone marrow imaging, as a method of functional imaging, can give in vivo information concerning the functional expression of the MDR phenotype in patients with untreated leukaemia.

Noninvasive detection of multidrug resistance in patients with hematological malignancies: are we there yet?

The success of chemotherapy in the treatment of malignancies may be limited by cellular mechanisms leading to drug resistance. In hematological malignancies, mechanisms leading to the development of multidrug resistance (MDR) include overexpression of the membrane-based export pump P-glycoprotein (Pgp) and the MDR-associated protein (MRP). Recently, the overexpression of the lung-resistance protein (LRP) has also been associated with reduced intracellular drug accumulation. A major problem in assessing the significance of the expression of these resistance proteins in clinical MDR has been the variability of detection techniques either at the mRNA or protein level. Currently, the detection of resistance proteins relies heavily on antibody and cDNA probes, and these methods may not be informative about the in vivo function of Pgp, MRP, or LRP. Nuclear medicine imaging techniques such as single-photon emission tomography (SPECT) and positron emission tomography (PET) have been evaluated for noninvasive determination of the presence and the function of Pgp- and MRP-mediated transport systems. Technetium 99m ((99m)Tc)-sestamibi, an agent in clinical use for myocardial perfusion and tumor imaging, is recognized as a substrate for Pgp and MRP, and has been used to visualize Pgp expression. (99m)Tc-tetrofosmin is also a substrate for the Pgp efflux pump mechanism and is used to evaluate Pgp function in in vitro and in vivo studies. Recently, radiopharmaceuticals including carbon 11-labeled colchicine, verapamil, and daunorubicin have been used in cell line and animal studies for the evaluation of Pgp-mediated transport functions using PET technology. Preliminary results suggest that the potential to detect MDR in tumors prior to or after exposure to chemotherapeutic agents exists in imaging using either (99m)Tc-labeled compounds and SPECT or positron emitting compounds and PET.