BRAF remains one of the most important therapeutic targets in cancer, but BRAF inhibitors can cause “paradoxical” pathway activation and drug resistance through RAF dimerization. A clinical “paradox breaker” inhibitor of BRAF monomers and dimers can potentially evade drug resistance. However, patients are required to receive a high oral daily drug dose to achieve the target therapeutic window. Co-administration of a cytochrome P450 blocker can improve drug exposure, but the combination can lead to drug-drug interactions. We investigated delivery via fucoidan-based nanocarriers to improve pharmacologic properties. We found that the nanoparticles extended BRAF inhibition in cancer cells due to sequestration into endolysosomes, followed by controlled release from a lysosomal depot. Following intraperitoneal administration, nanoparticles improved drug pharmacokinetics in vivo without inhibiting cytochrome P450 and also resulted in substantial improvements in anti-tumor efficacy. This work describes a general nanotherapeutic strategy to improve the pharmacologic properties of drugs via intracellular depot formation.