Compositions and methods for determining circulating biomolecules before, during, and/or after treatment of a patient with an anti-cancer or anti-tumor drug (or putative drug) are described. Methods of treatments based on the compositions and methods described herein are also provided. Noninvasive methods and kits are provided for assessing the efficacy of an anti-cancer therapy for killing or damaging cancer cells. Embodiments are used to determine the cancer-killing efficacy of an anti-cancer drug in a patient, to optimize the selection of an anti-cancer drug for treatment of a patient, to adjust the dosage of an anti-cancer drug for treatment of a particular cancer in a patient and for identifying useful anti-cancer therapeutics for any one particular type of cancer.