Is it possible that an anticancer drug IC50 value for a particular cancerous cell line decreases on conjugation with an antibody??
It is not only possible, but this is actually what you want to achieve by conjugating a cytotoxic drug to an antibody: create a "magic bullet" where antibody binding to a marker on the cell surface delivers the drug more efficiently to the cell (possibly after endocytosis). If that marker happens to be present on cancer, but not on normal cells, you have a drug candidate. Note also that conjugating the drug to an antibody may actually reduce its ability to passively enter cells not expressing the antigen, and hence reduce the side effects of the drug.
Let's say the IC50 was determined in vitro and then the drug was tested in vivo and the IC50 decreased due to some form of conjugation. It means the antibody potentiates the effect of the drug making it achieve its cytotoxic activity at lower concentrations of the parent drug. So, yes, it is possible. The popular observation would be to increase the IC50, but from experience, I wouldn't put anything past chemical reactions in biological systems. They largely remain a poorly understood test system.
It is not only possible, but this is actually what you want to achieve by conjugating a cytotoxic drug to an antibody: create a "magic bullet" where antibody binding to a marker on the cell surface delivers the drug more efficiently to the cell (possibly after endocytosis). If that marker happens to be present on cancer, but not on normal cells, you have a drug candidate. Note also that conjugating the drug to an antibody may actually reduce its ability to passively enter cells not expressing the antigen, and hence reduce the side effects of the drug.
What you have described is "synergy" and it is one of the more investigated issues in chemotherapy and in cancer treatment in general.
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