Doxorubicin (DOX) is currently used in cancer chemotherapy to treat many tumors and shows improved delivery, reduced toxicity and higher treatment efficacy when being part of nanoscale delivery systems. However, a major drawback remains its toxicity to healthy tissue and the development of multi-drug resistance during prolonged treatment. This is why in our work we aimed to improve DOX delivery and reduce the toxicity by chemical conjugation with a new nanoplatform based on polymalic acid. For delivery into recipient cancer cells, DOX was conjugated via pH-sensitive hydrazone linkage along with polyethylene glycol (PEG) to a biodegradable, non-toxic and non-immunogenic nanoconjugate platform: poly(beta-L-malic acid) (PMLA). DOX-nanoconjugates were found stable under physiological conditions and shown to successfully inhibit in vitro cancer cell growth of several invasive breast carcinoma cell lines such as MDA-MB-231 and MDA-MB-468 and of primary glioma cell lines such as U87MG and U251.