Isobologram Analysis
Also known as: isobol, combination index, synergy testing
Isobologram analysis is a graphical and quantitative method for detecting and classifying drug interactions, developed by Salvatore Loewe in 1926. It uses dose-response data from two drugs applied individually and in combination to determine whether their interaction is additive, synergistic, or antagonistic.
Key highlights
- Provides both visual (graphical) and quantitative (CI) assessment of drug interactions
- Relatively simple methodology applicable to diverse endpoints (cell viability, enzyme inhibition, antimicrobial activity)
- Combination Index (CI) is a standard, interpretable metric for comparing interactions across studies
- Reveals synergistic combinations that may be therapeutically valuable
Intuition
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How it works
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When to use it
Use isobologram analysis to evaluate combination therapies (e.g., drug cocktails, chemotherapy regimens) and to identify potential drug-drug interactions. It is standard in cancer research, microbiology, and toxicology for characterizing synergistic combinations.
Strengths & limitations
- Provides both visual (graphical) and quantitative (CI) assessment of drug interactions
- Relatively simple methodology applicable to diverse endpoints (cell viability, enzyme inhibition, antimicrobial activity)
- Combination Index (CI) is a standard, interpretable metric for comparing interactions across studies
- Reveals synergistic combinations that may be therapeutically valuable
- Assumes a fixed effect level; different effect levels may yield different interaction classifications
- Requires dense dose-response data for both single agents and multiple combinations, increasing experimental load
- Does not account for temporal effects or sequential dosing; assumes simultaneous drug application
- CI calculation assumes additivity model (Loewe); other models (Bliss, multiplicative) may yield different conclusions
Common pitfalls
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Applications
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Frequently asked
What does a Combination Index (CI) value tell me?
CI quantifies the degree of interaction. CI < 1 indicates synergy (combined effect stronger than expected). CI = 1 indicates additivity (combined effect equals sum of individual effects). CI > 1 indicates antagonism (combined effect weaker than expected). The further from 1, the stronger the interaction.
Should I use Loewe or Bliss model?
Loewe assumes drugs act through the same mechanism (additivity). Bliss assumes independent mechanisms (multiplicativity). For mechanistically similar drugs (e.g., two kinase inhibitors), Loewe is appropriate. For mechanistically distinct drugs, Bliss may be more suitable. Often both are calculated for comparison.
Can I use the same effect level for all drugs?
Yes, using a fixed effect level (e.g., IC50) is standard. However, exploring multiple effect levels (e.g., IC20, IC50, IC80) can reveal whether synergy is concentration-dependent.
How do I choose appropriate drug ratios for combination studies?
Equimolar ratios are simplest but not always most relevant. If one drug is more potent, use equipotent ratios (matching individual IC50 doses). Fixed ratios (1:1, 2:1) are also common. The choice should reflect the intended clinical or experimental context.
Sources
- 1.Loewe, S. (1926). Die Mischtoxizität. Zeitschrift für Experimentelle Pathologie und Therapie, 24, 315-334.
- 2.Bliss, C. I. (1939). The toxicity of poisons applied jointly. Annals of Applied Biology, 26(3), 585-615.
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Cite this page
ScholarGate. (2026, June 3). Isobologram Analysis. ScholarGate. https://scholargate.app/pharmacology/isobologram-analysis