Biological system-analysis by the mass-action law with biophysical, biochemical and mathematical induction-and-deduction leads to the derivation of >300 reaction rate equations. These derived equations led to general biodynamics/pharmacodynamics and bioinformatics (MAL-BD/PD/BI) general paradigms for “Dose” and “Effect”. 1. Median-Effect Eq. (MEE) for single drug or entity, regardless, in vitro, in animals or in human, where Dm and m values signify the median-effect dose “potency” and “shape” (dynamic order), respectively. 2. Combination Index Eq. (CIE), where CI<1, =1, and >1 indicates synergism, additive effect and antagonism, respectively, in combinations. Since all terms in MEE and CIE are relativity ratios, they are dimensionless quantities. Therefore, MEE and CIE are fundamental MAL-BD/PD/BI/CI principle independent to entity’s physical nature (chemicals, biologicals, natural products, radiation, liquid, or gas); independent to unit (ug, nM, mg/ml. mg/kg, IU, rad or multiples of infection); and independent to mode or mechanism of actions (competitive, noncompetitive, uncompetitive, sequential, ordered, ping-pong or random). Comprehensive details are available in Chou TC, Pharmacol. Rev. 58: 621-681, 2006 (specifically on p.655 and p.660), It also discussed in Chou, Integrative Biol. 3: 548-559, 2011, and many more recent papers on chemotherapy and radiation therapy combinations, as indicated in Google scholars. The above 2006 Chou article, as of January 12, 2021, has 3,835 citations in 1,054 biomedical journals internationally, encompassing nearly all disciplines in biomedical sciences.

Citation Format: Ting-Chao Chou. Why drug and radiation combination synergism or antagonism can be quantitatively determined with the mass-action law based pharmacodynamics equation, algorithm and computer simulation [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-062.