# Derivatization of Bufadienolides at Carbon-3 of the Steroid Core and Their Consequences for the Interaction with Na+,K+-ATPase

## TL;DR

This paper explores how chemical modifications at a specific site on steroid compounds affect their interaction with a key enzyme, which could help design better anticancer drugs with fewer side effects.

## Contribution

The study provides a mechanistic model explaining how structural changes at carbon-3 influence bufadienolide interactions with Na+,K+-ATPase.

## Key findings

- The optimal substituent size balances stabilizing interactions against steric and entropic interferences.
- N-methoxy linkers restrict rotation and place the steroid core in a less favorable position compared to ether bonds.
- Changing from β- to α-anomer reduces ligand contacts with the enzyme's binding site.

## Abstract

Bufadienolides exert broad-spectrum pharmacological activities relevant to cardiology and novel cancer treatments. Their efficacy, toxicity, and pharmacokinetic profiles are significantly affected by modifications at carbon-3 (C3) of the steroid core. We have applied molecular dynamics simulations to characterize the consequences of (i) variations in size of the substituent at C3, (ii) the type of linker at C3 (ether vs. N-methoxy), and (iii) stereochemistry (C3β vs. C3α) for derivatives’ interactions with Na+,K+-ATPase. The model compounds included bufalin, bufalin-N-glucose, bufalin-O-glucose as well as digoxigenin, digoxigenin monodigitoxoside and digoxin. It was shown that the optimal size of the substituent is a trade-off between the ability to form stabilizing interactions and steric and entropic interferences. The former is strongly affected by the nature of the linker due to its impact on the spatial position of the ligand: N-methoxy linker imposes rotational restrictions and places the core into a less favorable position compared to an ether bond. Similarly, the change from β- to α-anomer delocalizes the substituent precluding contacts with amino acid residues of the binding site. The presented mechanistic model of bufadienolide interactions with Na+,K+-ATPase helps to anticipate the consequences of modifications while designing derivatives with high anticancer activity but reduced cardiotoxicity.