Initial Stages of Spontaneous Binding of Folate-Based Vectors to Folate Receptor‑α Observed by Unbiased Molecular Dynamics

Active targeting is a prospective
strategy for controlled drug
delivery to malignant tumor tissues. One of the approaches relies
on recognition of a bioactive ligand by a receptor expressed abundantly
on the surface of cancer cell membranes. A promising ligand–receptor
pair is folic acid (or its dianionic form, folate) combined with the
folate receptor-α (FRα). A number of targeting drug delivery
systems based on folate have been suggested, but the mechanism of
binding of the ligand or its derivatives to the receptor is not fully
known at the molecular level. The current study summarizes the results
from unbiased all-atom molecular dynamics simulations at physiological
conditions describing the binding of two forms of folate and four
of its synthetically available derivatives to FRα. The models
(ca. 185,000 atoms) contain one receptor molecule, embedded in the
outer leaflet of a lipid bilayer, and one ligand, all immersed in
saline. The bilayer represents a human cancer cell membrane and consists
of 370 asymmetrically distributed lipid molecules from 35 types. The
ability of the vector molecules to bind to the receptor, the position
of binding, and the interactions between them are analyzed. Spontaneous
binding on the nanosecond scale is observed for all molecules, but
its time, position, and persistence depend strongly on the ligand.
Only folate, 5-methyltetrahydrofolate, and raltitrexed bind selectively
at the active site of the receptor. Two binding poses are observed,
one of them (realized by raltitrexed) corresponding qualitatively
to that reported for the crystallographic structure of the complex
folate-FRα. Pemetrexed adsorbs nonspecifically on the protein
surface, while methotrexate and pteroyl ornithine couple much less
to the receptor. The molecular simulations reproduce qualitatively
correctly the relative binding affinity measured experimentally for
five of the ligands. Analysis of the interactions between the ligands
and FRα shows that in order to accomplish specific binding to
the active site, a combination of hydrogen bonding, π-stacking,
and van der Waals and Coulomb attraction should be feasible simultaneously
for the vector molecule. The reported results demonstrate that it
is possible to observe receptor–ligand binding without applying
bias by representing the local environment as close as possible and
contain important molecular-level guidelines for the design of folate-based
systems for targeted delivery of anticancer drugs.