Fluoro-, bromo-, and iodopaclitaxel derivatives: synthesis and biological evaluation
Introduction
Paclitaxel (PAC, 1, Fig. 1) is a complex diterpene natural product that has application as a chemotherapeutic agent in a number of solid tumors. A semi-synthetic analog, docetaxel (2, Fig. 1), is also a clinically important chemotherapeutic agent. Georg et al. reviewed the basic science of taxanes, including structure activity relationships, metabolism, and pharmacology [13].
PAC exerts its antitumor activity by binding to β-tubulin, resulting in the disruption of cell division [4], [12], [19]. Unfortunately, PAC’s effectiveness for the treatment of some tumors has been reduced by the tumor cell’s ability to develop multidrug resistance (MDR). One major form of MDR results from enhanced expression of P-glycoprotein (P-gp), the product of the MDR1 gene found in normal human hepatocytes, epithelial cells of colon and jejunum, and renal proximal tubules [49]. P-gp effects the removal of a wide variety of cytotoxic substances, including PAC, by functioning as an ATP dependent efflux pump [1]. While this “drug pump” model may explain increased resistance to chemotherapeutic drugs, it does not account for other cellular changes observed in cells overexpressing P-gp; hence, another model that involves P-gp indirectly altering the partitioning of the chemotherapeutic drugs has been proposed [36].
We set out to develop a radiolabeled analogue of PAC for use as a positron emission tomography (PET) imaging agent for evaluating the expression of P-gp in certain tumor cell types. In multidrug resistant cell lines, a radiolabeled PAC analogue can be used to determine whether P-gp modulators alter the uptake of PAC. A positron-emitting PAC derivative could be used for the in vivo measurement of PAC concentration in tumors and predict, prior to chemotherapeutic treatment, the potential benefit to a patient. The effect of modulators on PAC therapy can be studied in individual patients. Such studies may lead to the development more effective therapies for the treatment of MDR tumors.
Other radiopharmaceuticals have been evaluated as imaging agents to study MDR [18], [11], [25]. 99mTc-sestamibi has been used to study P-gp modulation in vivo in the liver and kidney [27] and in mdr1a/1b knockout mice [46]. Del Vecchio et al. showed that both tumor uptake and clearance of this radiopharmaceutical correlated with P-gp expression [8]. 99mTc-Tetrofosmin has also been studied as a marker for P-gp function [2]. The synthesis of an indium-111 labeled analogue of PAC has also been reported in the literature [26]. This indium analogue demonstrated favorable tumor-to-blood and tumor-to-muscle ratios and was used to successfully image tumors in tumor-bearing mice. The synthesis of I-123-labeled PAC was recently reported, but biodistribution data were not included [37].
In this manuscript, we describe the synthesis of fluoro- (FPAC, 4, Fig. 2), bromo- (BPAC, 5, Fig. 2), and iodo- (IPAC, 6, Fig. 2) analogs of PAC. Each of these elements has a radionuclide (F-18, Br-76, and I-124) with suitable properties for in vivo imaging with PET. The analogues were prepared by selective acylation of the unsubstituted 3′ amine of paclitaxel primary amine (3, Fig. 2) without protection of the alcohol functions present in the molecule. We prepared fluorine-18, bromine-76, and iodine-124 analogues and studied their in vivo biodistributions in Sprague-Dawley rats. In addition, we conducted studies on the effect of preadminstration of PAC or XR9576 (Fig. 1), a P-gp modulator, on the biodistribution of the fluorine-18 analogue in mdr1a/1b(-/-) knockout mice.
Section snippets
General chemical methods
Paclitaxel {[2aR-[2aα, 4β, 4aβ, 6β, 9α(αR*,βS*), 11α, 12α, 12aα, 12bα]]-β-(benzoylamino)-α-hydroxybenzene-propanoic acid 6,12b-bis(acetyloxy)-12-benzoyloxy)-2a,3,4,4a,5,6,9,10,11,12,12a,12b-dodecahydro-4,11-dihydroxy-4a,8,13,13-tetramethyl-5-oxo-7,11-methano-1H-cyclodeca [3], [4]benz[1,2-b]oxet-9yl ester} (1, Fig. 1) and paclitaxel primary amine (3, Fig. 3, Fig. 4 were purchased from Hauser (Boulder, CO).1 Unless otherwise stated, all other chemical reagents and catalysts were purchased from
Chemistry
We synthesized fluoro (4), bromo (5), and iodo (6) analogues of PAC via acylation of 3 with the appropriate benzoyl chloride in CH2Cl2 with triethylamine (Fig. 2).
Radiochemistry
The radiosynthetic methodology for incorporation of F-18 (t -110 min) was based on our laboratory’s prior experience with [18F]fluorobenzoylchloride (10) and other acid chlorides (Fig. 3) [24]. The radiolabeling precursor (7) was previously described [23]. The radiofluorination of this precursor was achieved using aqueous [18
Chemistry
Earlier syntheses of paclitaxel derivates modified on the phenylisoserine moiety involved the synthesis of analogues of phenylisoserine or 3-hydroxy-4-phenyl-2-azetidinone and subsequent coupling with baccatin III (Fig. 1) [16]. The latter synthetic approach was utilized to synthesize 3′ N-debenzoylpaclitaxel (3). 3 has also been prepared from a suitably protected paclitaxel by direct debenzoylation [33]. With 3 in hand, these same authors utilized Schotten-Baumann conditions to acylate the
Conclusions
We prepared three halogenated analogues of paclitaxel and developed radiosyntheses of [18F]FPAC, [76Br]BPAC, and [124I]IPAC. Biodistribution studies were conducted to evaluate the potential utility in PET. While the uptake of [18F]FPAC and [76Br]BPAC was increased most significantly in rat kidney upon preinjection with PAC, studies in knockout mice suggest that the increased uptake cannot be attributed to inhibition of P-gp-mediated transport. Lung was the only tissue in which all three
Acknowledgements
The authors acknowledge Dr. Michael Gottesman and Dr. David Piwnica-Worms for extremely helpful and valuable discussions on the interpretation of biodistribution data. The authors thank Noel Whittaker of the Laboratory of Analytical Chemistry, NIDDK, for FAB-MS. We acknowledge the cyclotron group in the NIH Positron Emission Tomography Department for isotope production. The authors thank Josie Divel of the Positron Emission Tomography Department’s Quality Control section for specific activity
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