Surface modification of magnetic nanoparticles (MNPs) with tetramethylorthosilicate and 3-aminopropyltrimethoxysilane was studied. The use of N-phosphonomethyliminodiacetic acid for the stabilization of the initial MNPs can significantly increase the coating thickness and specific surface area of SiO2/APS-modified (APS is 3-aminopropylsilane) MNPs. The possibility of sorption of antitumor drug doxorubicin (DOX) on the synthesized nanoparticles was studied. The degree of sorption of DOX increases with an increase in the specific surface area of the particles.