Limitations of (99m)Tc tetrofosmin in assessing reversal effects of verapamil on the function of multi-drug resistance associated protein 1.
BACKGROUND
Previous reports have demonstrated the feasibility of scintigraphic assessment of the multi-drug resistance (MDR) of tumours caused by ATP binding cassette (ABC) transporters by using Tc cationic tracers such as Tc tetrofosmin (TF). Furthermore, the potential of these tracers for evaluating the effects of reversal agents for MDR has been documented. However, most reversal agents simultaneously affect cationic ion transporters related to tracer accumulation in tumours.


METHODS
The uptake of Tc-TF was examined in the MCF7/WT cell line, a wild-type breast cancer cell line that does not exhibit MDR, and its subclonal etoposide resistant cell line MCF7/VP, which expresses high levels of MRP1, one of the multi-drug resistance associated proteins (MRPs), in the presence of increasing concentrations of verapamil, a classical MDR modulator. In the absence of verapamil, MCF7/VP cells showed significantly lower Tc-TF uptake than did MCF7/WT cells, indicating that Tc-TF is a substrate for MRP1. The presence of verapamil enhanced the uptake of Tc-TF in MCF7/VP cells. On the other hand, verapamil also increased tracer uptake in MCF7/WT cells, which was readily appreciated when the uptake values were corrected by viable cell numbers: an approximately 100% increase of Tc-TF uptake was observed in comparison with that in the absence of verapamil in viable MCF7/WT cells whereas a 100-200% increase occurred in viable MCF7/VP cells. In addition, verapamil prolonged the retention of radioactivity in both MCF7/WT cells and MCF7/VP cells.


CONCLUSIONS
These results suggest that cellular functions other than MRP1 function, probably cationic ion transporters, are simultaneously and significantly involved in the verapamil induced changes of cellular uptake of Tc-TF. Tc-TF scintigraphy may overestimate the reversal effects of modulators on chemoresistance caused by MRP1 when the modulators simultaneously affect ion transporters.
Kinuya
Seigo
S
Department of Biotracer Medicine, Kanazawa University Graduate School of Medical Sciences, Kanazawa, Japan. kinuya@med.kanazawa-u.ac.jp
Li
Xiao-Feng
XF
Yokoyama
Kunihiko
K
Mori
Hirofumi
H
Shiba
Kazuhiro
K
Watanabe
Naoto
N
Shuke
Noriyuki
N
Bunko
Hisashi
H
Michigishi
Takatoshi
T
Tonami
Norihisa
N
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000008:administration & dosage; D001943:Breast Neoplasms / Q000000981:diagnostic imaging* / Q000188:drug therapy / Q000378:metabolism*; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D018432:Drug Resistance, Multiple / Q000187:drug effects*; D006801:Humans; D009943:Organophosphorus Compounds / Q000493:pharmacokinetics*; D015609:Organotechnetium Compounds / Q000493:pharmacokinetics*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D016896:Treatment Outcome; D014700:Verapamil / Q000008:administration & dosage*
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000970:Antineoplastic Agents; D009943:Organophosphorus Compounds; D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals; C078700:technetium tc-99m tetrofosmin; D014700:Verapamil