Reducing safety-related drug attrition: the use of in vitro pharmacological profiling

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In vitro pharmacological profiling is increasingly being used earlier in the drug discovery process to identify undesirable off-target activity profiles that could hinder or halt the development of candidate drugs or even lead to market withdrawal if discovered after a drug is approved. Here, for the first time, the rationale, strategies and methodologies for in vitro pharmacological profiling at four major pharmaceutical companies (AstraZeneca, GlaxoSmithKline, Novartis and Pfizer) are presented and illustrated with examples of their impact on the drug discovery process. We hope that this will enable other companies and academic institutions to benefit from this knowledge and consider joining us in our collaborative knowledge sharing.

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Figure 1: Alignment of in vitro pharmacology profiling to the drug discovery and development process.
Figure 2: Levels of promiscuity among marketed drugs and Novartis's compounds.
Figure 3: In vitro profiling during lead optimization.


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The authors thank the following individuals for their valuable discussions: J.-P. Valentin and C. Pollard from AstraZeneca; L. Urban, P. Muller and G. Erdemli from Novartis; N. McMahon and J. Louttit from GlaxoSmithKline; and A. Mead from Pfizer.

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Correspondence to Joanne Bowes or Andrew J. Brown or Jacques Hamon or Wolfgang Jarolimek or Arun Sridhar or Gareth Waldron or Steven Whitebread.

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Concentration required to elicit a 50% response in an in vitro assay. IC50 refers to an inhibitory response (the half maximal inhibitory concentration) and EC50 refers to an effect (the effector concentration for half-maximum response), usually an activation or stimulation. AC50 is a collective term used for any activity.

Adverse drug reactions

(ADRs). Any noxious, unintended and undesired effects of a drug, occurring at doses used in humans for prophylaxis, diagnosis or therapy. These exclude therapeutic failures, intentional and accidental poisoning and drug abuse.


The concentration of an agonist that is required to produce 50% of the maximum response of that agonist.

Free Cmax

The fraction of the Cmax (peak total plasma concentration of a drug at a certain dose) that is not bound to plasma proteins. The percentage of the bound drug is determined separately and the Cmax is corrected accordingly.


The half maximal inhibitory concentration, or the concentration of an inhibitor that is required for 50% inhibition of the maximum control response in a biochemical or cellular assay.

K i

Inhibition constant; can be derived from the IC50 (half maximal inhibitory concentration) if the concentration of ligand or substrate and its dissociation or Michaelis constant is known. Should be used in preference to IC50 for binding assays.

Safety margins

Ratios of an AC50 (concentration required to elicit a 50% response in an in vitro assay) — or the inhibition constant Ki — of a drug at a target known to mediate specific adverse drug reactions (ADRs) and the therapeutic free plasma concentration. The latter can be directly determined in preclinical or clinical studies, or estimated from models. The AC50 is taken from the most relevant assay available for that target. Safety margins should be used as early as possible in the preclinical phase to continually assess the risk of an ADR occurring in the clinic.


The ratio of the AC50 (concentration required to elicit a 50% response in an in vitro assay) — or the inhibition constant Ki if available — of a drug at any target that is known or suspected to mediate an adverse drug reaction, and the primary (therapeutic) target.

Therapeutic free plasma concentration

The concentration of a compound in the plasma following a therapeutic dose. Often quoted as the maximum exposure.

Therapeutic index

In a drug development setting: the quantitative ratio of the exposure level at the chosen safety end point divided by the exposure level at the chosen efficacy end point, typically the ratio of the highest exposure to the drug that results in no toxicity over that which produces the desired efficacy. This term is often used incorrectly to describe the safety margin.

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