Multidrug resistance (MDR) that develops during chemotherapy desensitizes the tumor to a variety of structurally and functionally different chemotherapeutic agents and is linked to the expression of ATP-dependent drug efflux transporters such as MDR1 P-glycoprotein and MRP. MDR can be detected in vitro with molecular and functional assays such as protein, PCR, and FACS. However clinically we need to monitor tumor MDR status at different points during the therapy that would require multiple biopsies. In vivo nuclear imaging of MDR was demonstrated with SPECT using 99mTc-sestamibi, an MDR substrate, and PET using 11C-labeled analogs of anticancer drugs. SPECT imaging has low spatial resolution and multiple studies may require a substantial dose of radioisotopes. PET provides an improved image quality however the short lifetime of PET probes can limit feasibility of this technique. We developed a novel agent to probe MDR resistance in cells and animal tumor models that is compatible with noninvasive imaging such as MRI. The imaging agent consists of three functional domains as shown in the figure. The first domain, the amphiphilic HIV1 Tat membrane translocation peptide, facilitates internalization of the agent; the second domain, GdDOTA complex, generates T1 MR contrast, and the third domain, rhodamine, an MDR substrate, provides specific efflux of the agent from the drug resistant cell. The agent was synthesized using a standard solid-state peptide synthesis. We compared uptake of the compound in wild type MCF-7wt and drug resistant MCF-7adr breast cancer cells using fluorescent detection of rhodamine by microscopy and FACS. A significantly reduced accumulation of the agent was detected in the drug resistant cells. Treatment of MCF-7adr with 6 ug/ml cyclosporine resulted in reversal of the effect. T1-weighted spin-echo MR imaging of fixed cell pellets was performed on 11.7T Bruker scanner and demonstrated (i) significant positive signal enhancement in both MCF-7wt and MCF-7adr cells and (ii) more pronounced contrast uptake in the wild type cells. Development of a clinically applicable MDR imaging agent would be of utmost importance for predicting effective anticancer chemotherapy. This study was supported by NIH R01 CA097310 grant.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]