Delivering Phytochemicals to the Implantation Window: The Dual Constraint of Achieving Endometrial Exposure Without Compromising the Embryo

Whether a phytochemical promotes or suppresses endometrial receptivity is context-dependent, and the dose axis of that context cannot be placed on a concentration scale without knowing how much compound reaches the endometrium-a measurement this review's documented search did not retrieve for the human endometrium. This review asks why oral administration does not produce interpretable endometrial exposure, which routes can, and what safety constraint follows from targeting an endometrium that contains an embryo. Poor oral bioavailability is not a single obstacle: absorption failure, loss of the parent compound, intestinal conjugation, systemic metabolism and host-dependent speciation overlap, and what differs by class is which step dominates. Endometrial targeting is not unprecedented-vaginal progesterone attains endometrial concentrations disproportionate to serum through the first uterine pass, quantified in human tissue-but that precedent is specific to a small lipophilic steroid and to the released drug rather than the carrier. Uniquely, this target is dual-constrained: a carrier must reach the tissue without exposing an embryo that lipid nanoparticles reach by crossing the zona pellucida. Peri-implantation delivery therefore requires context-dependent design choices: release aligned to a defined window, bounded exposure, and separate verification of carrier distribution, released-drug exposure, endometrial effect and developmental outcome.