Folate-conjugated organic CO prodrugs: Synthesis and CO release kinetic studies

Abstract

Carbon monoxide (CO) is an endogenous produced molecule and has shown efficacy in animal models of inflammation, organ injury, colitis and cancer metastasis. Because of its gaseous nature, there is a need for developing efficient CO delivery approaches, especially those capable of targeted delivery. In this study, we aim to take advantage of a previously reported approach of enrichment-triggered prodrug activation to achieve targeted delivery by targeting the folate receptor. The general idea is to exploit folate receptor-mediated enrichment as a way to accelerate a biomolecular Diels-Alder reaction for prodrug activation. In doing so, we first need to find ways to tune the reaction kinetics in order to ensure minimal rection without enrichment and optimal activation upon enrichment. In this feasibility study, we synthesized two diene-dienophile pairs and studied their reaction kinetics and ability to target the folate receptor. We found that folate conjugation significantly affects the reaction kinetics of the original diene-dienophile pairs. Such information will be very useful in future designs of similar targeted approaches of CO delivery.

Keywords: Carbon Monoxide; Diels-Alder reaction; Enrichment triggered delivery; Folate conjugate; Reaction kinetics.

Figure 1

Design of organic CO prodrugs.[53]

Figure 2

Enrichment triggered delivery of CO by folate (A), and TPP (B) conjugates. C) Structures of folate and TPP conjugates used in Fig. A-B (“Created with BioRender.com”)

Figure 3

CO release kinetic studies of folate-conjugated prodrug partners (19a/19b, 11), and control compounds (20, 21) by using turn-on fluorescence of the product.

Figure 4

Determination of the pseudo-first order and second order rate constants for the Diels-Alder reactions. A-C) cycloalkyne (21) and cyclopentadienone (20); E-G cycloalkyne (11) and cyclopentadienone (19b). Fluorescent intensity was monitored by incubating different concentrations of cyclopentadienone and a fixed concentration of cycloalkyne (50 μM) with excitation and emission wavelength of 375 and 465 nm respectively. D, H) Second-order rate constant between cycloalkyne and cyclopentadienone.