Background

[PubMed]

One of the mechanisms that tumor cells use to escape the cytotoxic effects of chemotherapeutic agents, such as Adriamycin, Vinca alkaloids, epipodophyllotoxins, actinomycin D, and paclitaxel (PAC), is to limit their presence inside the cells through the actions of P-glycoprotein (P-gp), a protein encoded by the multidrug resistance (MDR-1) gene (1, 2). P-gp is an ATP-dependent transmembrane multidrug transporter that is capable of actively pumping a variety of agents out of cells. Injection of unlabeled efflux pump substrates increases the retention of radioactivity in tumors rather than lessening the retention, as seen with receptor-binding radiotracers. Overexpression of P-gp in tumor cells (such as renal carcinoma, hepatoma, pheochromocytoma, and colon carcinoma) leads to resistance to anticancer drugs (3). P-gp is also present in a variety of normal cells, such as intestinal mucosal cells, hepatocytes, renal proximal tubule epithelial cells, and endothelial cells of the blood-brain barrier (4, 5). Calcium channel blockers, cyclosporin A, and its non-immunosuppressive analog PSC 833 are MDR modulators that inhibit transport of P-gp substrates out of cells (6, 7).

Sestamibi (MIBI) is a substrate for P-gp. ^99mTc-MIBI has been approved by the United States Food and Drug Administration (FDA) as a myocardial perfusion imaging agent for use with single-photon emission computed tomography (SPECT) to assess the risk of future cardiac events. It is also approved as a tumor-imaging agent in breast, lung, thyroid, and brain cancers. PAC is an FDA-approved chemotherapeutic agent exerting its antitumor activity by binding to β-tubulin to inhibit cell division (8). It is also a transport substrate for P-gp in tumor cells, leading to drug-related resistance to chemotherapy (8, 9). Therefore, [¹⁸F]fluoropaclitaxel ([¹⁸F]FPAC) is being developed as a positron emission tomography (PET) agent to noninvasively study P-gp function and multidrug resistance in tumors and normal tissues.

Synthesis

[PubMed]

Kiesewetter et al. (10) prepared [¹⁸F]FPAC by incorporating [¹⁸F]fluoride (Kryptofix 2.2.2 and K₂CO₃) into [¹⁸F]fluorobenzoate ester via nucleophilic displacement of a trimethylammonium moiety. The ester group was removed, and the resulting [¹⁸F]fluorobenzoic acid was coupled to 3´-debenzoylpaclitaxel (10).The final product was purified by high-performance liquid chromatography. The radiochemical yield for the syntheses was 18.3 ± 5.5% (not corrected for decay). The specific activity at the end of bombardment was 169-453 GBq/mmol (4.58-12.25 Ci/mmol) for 14 syntheses. The total synthesis time was 80 min.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In an in vitro study using human hepatocytes, 50% [¹⁸F]FPAC was metabolized to produce one major metabolite, 6α-hydroxy-FPAC, after 4 h of incubation (10). In rat hepatocytes, 3 metabolites were produced from [¹⁸F]FPAC with a half-life of 194 min.

Animal Studies

Human Studies

[PubMed]

Kurdziel et al. (13) estimated the human dosimetry of [¹⁸F]FPAC in 3 healthy subjects after injection of 192.4 MBq (5.2 mCi) of [¹⁸F]FPAC. The organs receiving the highest radiation dose were the gallbladder, large intestine and small intestine at 0.23 mGy/MBq (0.85 rad/mCi), 0.19 mGy/MBq (0.68 rad/mCi), and 0.16 mGy/MBq (0.60 rad/mCi), respectively. The effective dose was 28.79 μSv/MBq (0.107 rem/mCi). [¹⁸F]FPAC PET studies were performed with dynamic scans for 60 min and static scans for 120 min in 3 breast cancer patients. The tumor uptake was low with an average maximum standard uptake value (SUV_max) of 1.8 at 80 s after injection decreased slightly over time. When compared with background tissue (contralateral breast), the tumors were visible with an average maximum tumor/background ratio of 7.7 at 20 min. Tumor/blood ratios increased slightly over time to an average maximum of 1.9.

NIH Support

Intramural Research Program, 1R21 CA098334-01A1

References

1.

Endicott J.A., Ling V. The biochemistry of P-glycoprotein-mediated multidrug resistance. Annu Rev Biochem. 1989;58:137–71. [PubMed: 2570548]

2.

Gottesman M.M., Pastan I. Biochemistry of multidrug resistance mediated by the multidrug transporter. Annu Rev Biochem. 1993;62:385–427. [PubMed: 8102521]

3.

Fojo A.T., Ueda K., Slamon D.J., Poplack D.G., Gottesman M.M., Pastan I. Expression of a multidrug-resistance gene in human tumors and tissues. Proc Natl Acad Sci U S A. 1987;84(1):265–9. [PMC free article: PMC304184] [PubMed: 2432605]

4.

Piwnica-Worms D., Rao V.V., Kronauge J.F., Croop J.M. Characterization of multidrug resistance P-glycoprotein transport function with an organotechnetium cation. Biochemistry. 1995;34(38):12210–20. [PubMed: 7547962]

5.

Thiebaut F., Tsuruo T., Hamada H., Gottesman M.M., Pastan I., Willingham M.C. Cellular localization of the multidrug-resistance gene product P-glycoprotein in normal human tissues. Proc Natl Acad Sci U S A. 1987;84(21):7735–8. [PMC free article: PMC299375] [PubMed: 2444983]

6.

Hughes C.S., Vaden S.L., Manaugh C.A., Price G.S., Hudson L.C. Modulation of doxorubicin concentration by cyclosporin A in brain and testicular barrier tissues expressing P-glycoprotein in rats. J Neurooncol. 1998;37(1):45–54. [PubMed: 9525837]

7.

Mayer U., Wagenaar E., Dorobek B., Beijnen J.H., Borst P., Schinkel A.H. Full blockade of intestinal P-glycoprotein and extensive inhibition of blood-brain barrier P-glycoprotein by oral treatment of mice with PSC833. J Clin Invest. 1997;100(10):2430–6. [PMC free article: PMC508442] [PubMed: 9366556]

8.

Horwitz S.B., Cohen D., Rao S., Ringel I., Shen H.J., Yang C.P. Taxol: mechanisms of action and resistance. J Natl Cancer Inst Monogr. 1993;(15):55–61. [PubMed: 7912530]

9.

Huang Y., Ibrado A.M., Reed J.C., Bullock G., Ray S., Tang C., Bhalla K. Co-expression of several molecular mechanisms of multidrug resistance and their significance for paclitaxel cytotoxicity in human AML HL-60 cells. Leukemia. 1997;11(2):253–7. [PubMed: 9009089]

10.

Kiesewetter D.O., Jagoda E.M., Kao C.H., Ma Y., Ravasi L., Shimoji K., Szajek L.P., Eckelman W.C. Fluoro-, bromo-, and iodopaclitaxel derivatives: synthesis and biological evaluation. Nucl Med Biol. 2003;30(1):11–24. [PubMed: 12493538]

11.

Gangloff A., Hsueh W.A., Kesner A.L., Kiesewetter D.O., Pio B.S., Pegram M.D., Beryt M., Townsend A., Czernin J., Phelps M.E., Silverman D.H. Estimation of paclitaxel biodistribution and uptake in human-derived xenografts in vivo with (18)F-fluoropaclitaxel. J Nucl Med. 2005;46(11):1866–71. [PubMed: 16269601]

12.

Kurdziel K.A., Kiesewetter D.O., Carson R.E., Eckelman W.C., Herscovitch P. Biodistribution, radiation dose estimates, and in vivo Pgp modulation studies of 18F-paclitaxel in nonhuman primates. J Nucl Med. 2003;44(8):1330–9. [PubMed: 12902425]

13.

Kurdziel K.A., Kalen J.D., Hirsch J.I., Wilson J.D., Bear H.D., Logan J., McCumisky J., Moorman-Sykes K., Adler S., Choyke P.L. Human dosimetry and preliminary tumor distribution of 18F-fluoropaclitaxel in healthy volunteers and newly diagnosed breast cancer patients using PET/CT. J Nucl Med. 2011;52(9):1339–45. [PMC free article: PMC3224978] [PubMed: 21849404]