Synthesis and radioiodination of two Cdk4 inhibitors.

Aim:

Accurate cell proliferation is depending on the correct execution of the cell cycle. The cell cycle is controlled and coordinated by various enzymes and proteins like cyclins, cyclin-dependent kinases (Cdk) and cyclin-dependent kinase inhibitors (Cki). Many tumors show dysregulated Cdk activity and/or Cdk expression, and in 80 % of tumors the cell cycle relevant Cdk4-D1/INK4/pRb/E2F signal cascade is altered. Hence, appropriately radiolabeled Cdk4 inhibitors may be promising molecular probes for imaging cell proliferation processes and tumor visualization by PET. This work describes the design and synthesis of two ¹²⁴I-labeled Cdk4 inhibitors 8-cyclopentyl-6-[¹²⁴I]iodo-5-methyl-2-(4-piperazin-1-yl-phenylamino)-8H-pyrido[2,3-d]pyrimidin-7-one ([¹²⁴I]CKIA) and 8-cyclopentyl-6-[¹²⁴I]iodo-5-methyl-2-(5-(piperazin-1-yl)pyridin-2-ylamino)-8H-pyrido[2,3-d]pyrimidin-7-one ([¹²⁴I]CKIB) (figure) as potential radiotracers for imaging Cdk4 expression in vivo.

Results:

Reference substances and labeling precursors were prepared with minor modifications according to the chemistry described by Barvian et al.¹ and VanderWel et al.². The trimethylstannane compounds CKIAP and CKIBP as labeling precursors for the radioiodination reaction with I-124 were prepared in a multi-step synthesis sequence. Radioiodination was achieved by regioselective destannylation reaction under mild conditions using [¹²⁴I]NaI and Chloramine-T or Iodogen as oxidizing agents. Treatment of a solution (50 µl) containing CKIAP (5 mg/ml) with [¹²⁴I]NaI (36 MBq) in Iodogen precoated tubes afforded 12 MBq (34 %, decay-corrected) of [¹²⁴I]CKIA, including HPLC purification. The optimal reaction conditions for the [¹²⁴I]CKIA synthesis were applied to the synthesis of [¹²⁴I]CKIB. Starting from a 50 µl solution of CKIBP (5 mg/ml) and [¹²⁴I]NaI (29 MBq), compound [¹²⁴I]CKIB was obtained in a 34 % decay-corrected radiochemical yield, including HPLC purification. Both compounds were isolated in radiochemical purity exceeding 95 % and high chemical purity. The specific activities were determined to be 20 GBq/µmol and 4 GBq/µmol, respectively. The lipophilicity (logP) was experimentally measured. The logP value was 2.77 ± 0.13 for [¹²⁴I]CKIA and 1.99 ± 0.03 for [¹²⁴I]CKIB.

Conclusion:

The radiosynthesis of two ¹²⁴I-labeled Cdk4 inhibitors has been developed. Both radiotracers were obtained in reproducible radiochemical yields and purity enabling further radiopharmacological characterization aimed at imaging Cdk4 expression by means of PET.

References:

1 : Barvian, M.; Boschelli, D. H.; Cossrow, J.; Dobrusin, E.; Fattaey, A.; Fritsch, A.; Fry, D.; Harvey, P.; Keller, P.; Garrett, M.; La, F.; Leopold, W.; McNamara, D.; Quin, M.; Trumpp-Kallmeyer, S.; Toogood, P.; Wu, Z. P.; Zhang, E. L. Journal of Medicinal Chemistry, 2000, 43, 4606-4616.
2 : VanderWel, S. N.; Harvey, P. J.; McNamara, D. J.; Repine, J. T.; Keller, P. R.; Quin, J.; Booth, R. J.; Elliott, W. L.; Dobrusin, E. M.; Fry, D. W.; Toogood, P. L. Journal of Medicinal Chemistry, 2005, 48, 2371-2387.