Author ORCID Identifier
Doctor of Philosophy
Martin K. Safo
Introduction: Allosteric effectors of hemoglobin (AEH) represent a class of synthetic aromatic aldehydes that transiently form covalent interactions (Schiff-base) with hemoglobin (Hb) to form Hb-AEH adduct, preventing the HbS polymerization and sickling of red blood cells (RBC). The overall objective of this research was to aid in the optimization of novel AEH by understanding their target-site disposition of AEH in relevant biological matrices, e.g., HbA solution, whole blood (WB) and human liver cytosol (HLC), a surrogate of aldehyde dehydrogenase (ALDH)-mediated oxidative metabolism.
Methods: A “universal” HPLC-UV/Vis assay method was developed for the quantitation of HbA-AEH adduct for chemically different AEH in HbA solution and in WB. Steady-state (SS) concentration-dependence and time- dependence HbA binding were characterized by a sigmoidal Bmax-model (KD) and a kinetic binding model (kon, koff). The Warburg method was used to assess the enzymatic oxidation of selected AEH in HLC. WB disposition of 5-HMF (VCU lead, terminated in Phase 2) and VZHE-039 (current top candidate, pre-IND) was assessed by measuring their HbA-AEH adduct in WB and plasma exposures.
Results: All vanillin-derived benzaldehydes exhibit enhanced binding affinity in HbA solution, primarily due to their faster kon. Across AEH, i.e., benzaldehydes and furaldehydes studied, kon and koff values are strongly (log-log) correlated (r=0.993, n=7), suggesting that the molecular modifications of the current AEH scaffold enhance their interactions with Hb (as intended), but also (inadvertently) increase their Hb-AEH adduct dissociation.
In HLC, some AEH showed oxidative metabolism (CLint: 5-HMF < TD-7 < INN-310 < acetaldehyde, prototypical substrate). Presence of the meta-methoxy group (in TD-7) favors ALDH-mediated oxidation, while the ortho-hydroxy group (in VZHE-039) protects the aldehyde group/AEH from metabolism.
In WB, Hb-AEH adduct concentrations for VZHE-039 are sustained in WB for 24 hrs, confirming its resistance to ALDH metabolism in RBC. A less than proportional increase in plasma cmax, but proportional AUC along with prolonged tmax with the initial concentration, [VZHE-039]0, were observed for VZHE-039, indicating rate-limiting, but ultimately complete dissolution, prior to any HbA binding. The estimated apparent binding affinity (1/KDkinetic’) for VZHE-039 in WB is 23-fold lower than observed in HbA solution, primarily due to a 300-fold slower apparent kon’. On the other hand, HbA-AEH adduct concentrations for 5-HMF in WB decrease after 1.5 hrs - thus providing conclusive evidence of rapid intra-RBC ALDH-mediated metabolism - which resulted in an 8-fold lower apparent binding affinity (KD’ of 3.0 mM) from concentration-dependency study at 1.5 hrs in WB. However, after accounting for first-order metabolism in the WB disposition model, the KDkinetic’ for 5-HMF in WB was quite similar to that in HbA solution; dissociation of 5-HMF from Hb was slow and determined the terminal decline of HbA-AEH adduct in WB.
Conclusions: WB disposition of AEH is determined by the kinetics of three concurrent processes, namely (saturable) Hb binding, (non-saturable) ALDH-mediated metabolism in RBC, as well as reversible, first-order PPB/RBC membrane binding. Although the KD value for VZHE-039 is 3-fold lower than 5-HMF in HbA solution, both compounds exhibit similar apparent KD’ values in WB, albeit due to different mechanisms, namely ALDH-mediated metabolism for 5-HMF and extensive PPB/RBC membrane binding for VZHE-039 – both processes reducing AEH concentrations at the target site, Hb. Concurrent PPB/RBC membrane binding for lipophilic benzaldehydes, e.g., VZHE-039, also slows down their overall WB HbA adduct formation, but helps sustain them.
Key molecular features of AEH to enhance HbA binding and reduce intra-RBC metabolism have been identified, allowing further rational design of more potent (in WB) drug candidates for further druggability evaluation - with VZHE-039 as the top candidate.
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Available for download on Friday, August 06, 2021