  A key factor influencing a drug's efficacy is its residence time in the
binding pocket of the host protein. Using atomistic computer simulation to
predict this residence time and the associated dissociation process is a
desirable but extremely difficult task due to the long timescales involved.
This gets further complicated by the presence of biophysical factors such as
steric and solvation effects. In this work, we perform molecular dynamics (MD)
simulations of the unbinding of a popular prototypical hydrophobic
cavity-ligand system using a metadynamics based approach that allows direct
assessment of kinetic pathways and parameters. When constrained to move in an
axial manner, we find the unbinding time to be on the order of 4000 sec. In
accordance with previous studies, we find that the ligand must pass through a
region of sharp dewetting transition manifested by sudden and high fluctuations
in solvent density in the cavity. When we remove the steric constraints on
ligand, the unbinding happens predominantly by an alternate pathway, where the
unbinding becomes 20 times faster, and the sharp dewetting transition instead
becomes continuous. We validate the unbinding timescales from metadynamics
through a Poisson analysis, and by comparison through detailed balance to
binding timescale estimates from unbiased MD. This work demonstrates that
enhanced sampling can be used to perform explicit solvent molecular dynamics
studies at timescales previously unattainable, obtaining direct and reliable
pictures of the underlying physio-chemical factors including free energies and
rate constants.
