Design, synthesis and biological evaluation of a highly-potent and cancer cell selective folate-taxoid conjugate

Abstract

The folate receptor (FR) has been widely recognized as an excellent target for the tumor-selective delivery of cytotoxic agents, and four folate-drug conjugates have entered clinical evaluations for the treatment of solid tumors to date. However, most of these conjugates required structural modification of the cytotoxic warheads in order to achieve efficient drug release from the linkers. We designed and constructed a novel folate conjugate of a highly-potent next-generation taxoid, SB-T-1214, by exploiting bioorthogonal Cu-free 'click' chemistry. The synthesis was highly convergent and required no HPLC purification to obtain the final folate-taxoid conjugate 1. Conjugate 1 demonstrated highly FR-specific potency (IC₅₀ 2.1-3.5 nM) against a panel of cancer cell lines, with a >1000-fold decrease in cytotoxicity against normal human cells (IC₅₀>5000 nM). The remarkable potency and selectivity of conjugate 1 can be attributed to highly FR-specific receptor-mediated endocytosis as well as efficient release of the unmodified cytotoxic warhead using a mechanism-based self-immolative linker.

Keywords: Anticancer agent; Drug conjugate; Folic acid; Taxoid; Tumor-targeted.

Figure 1

Mechanism of the self-immolative disulfide linker.

Figure 2

Retrosynthetic analysis for the folic acid conjugate of next-generation taxoid SB-T-1214 (1)

Scheme 1

Synthesis of folyl(dipeptide)-PEG-azide 2 Reagents and Conditions: a) 11-azido-3,6,9-trioxa-11-undecan-1-amine, EDC^·HCl, CH₂Cl₂, 83%; b) TFA, CH₂Cl₂, 98%; c) piperidine, DMF; d) 5, HOBt, HBTU, DMF; e) piperidine, DMF; f) Fmoc-Glu(OH)-OtBu, HOBt, HBTU, DMF; g) piperidine, DMF; h) N¹⁰-TFA-pteroic acid, HOBt, HBTU, DMF; i) TFA, TIPS, H₂O, 56%; j) NH₄ OH, H₂O, quant.

Scheme 2

Synthesis of amino-PEG₃-cyclooctyne 11 Reagents and Conditions: a) LiOH, H₂O, dioxane, 50 °C, 91%; b) NHS, DIC, DMAP, CH₂Cl₂, 83% c.) 4,7,10-trioxa-1,13-tridecanediamine, CH₂Cl₂, 73%.

Scheme 3

Synthesis of folate-taxoid conjugate 1