Binding Kinetics

In biochemical and cellular assays performed at equilibrium, the drug and target are present at constant concentrations and thus the IC₅₀ accurately reflects the concentration of the drug-target complex. However, in the body the concentration of a drug constantly changes due to several physiological processes. Hence, affinity is not the only relevant parameter that predicts target occupancy and ultimately efficacy: clear understanding of binding kinetics provides a more complete picture of what drives target occupancy in
vivo^1-6.

The correlation of in vivo efficacy with residence time has been demonstrated across a spectrum of target types.

Comprehensive reviews of the importance of drug-target kinetics

1. Copeland R.A. (2016) Nat Rev Drug Discov. 15(2):87–95.
2. Doris A Schuetz D.A. (2017) Drug Discov Today. 22(6):896-911.
3. Swinney D.C. (2004) Nat Rev Drug Discov. 3(9):27–41.
4. Zhang R. et al. (2010) Expert Opin Drug Discov. 5(11):1023-9.
5. Tummino P.J. et al. (2008) Biochemistry. 47(20):5481–5492.
6. Lu H. at al. (2010) Curr Opin Chem Biol. 14(4):467–474.

There are many medicines for which binding kinetics contribute to efficacy and duration of action. One example is Candesartan, an antihypertensive drug that lowers blood pressure more efficiently than any other member of the same class, such as Losartan.

While Candesartan and Losartan share the same binding site, similar pharmacokinetics and receptor occupancy profiles, their binding kinetics are completely different^1-4. In a clinical trial study, the long-lasting pharmacodynamic effect of Candesartan provided greater efficacy at a lower dose (16 mg) than Losartan (100 mg) and maintained efficacy in the event of a missed dose⁵.

Left: Dose–response curves for the effect of Candesartan (orange) and Losartan (blue) on blood pressure in hypertensive patients. Right: Pharmacodynamic, pharmacokinetic and clinical features of Candesartan and Losartan. Adapted from Elmfeldt D. et al (2002).

References

1. Vauquelin G. et al (2010) Br J Pharmacol. 161(3): 488–508.
2. Tummino P.J. et al. (2008) Biochemistry. 47(20):5481–5492.
3. Copeland R.A. et al. (2006) Nat Rev Drug Discov 5(9):730-739.
4. Swinney D.C. (2004) Nat Rev Drug Discov. 3(9):27–41.
5. Elmfeldt D. et al (2002) Blood Press. 11(5):293-301.

Simple affinity measurements may mask the true impact of structural changes made in the same chemical series.

Ayaz and colleagues show this with a series of Roniciclib analogs of CDK2¹. They found that Roniciclib, the trifluoromethyl derivatives (CF₃) and bromo analogues (Br) showed similar kinase and cell proliferation inhibition along with comparable pharmacokinetic profiles. However, Roniciclib and the CF₃ derivatives had longer residence times and superior efficacy in tumor growth inhibition relative to the corresponding bromo analogues.

Left: Comparison of the residence times of compounds 1–5 and Roniciclib. The CF₃ derivatives (orange) have up to 15 times longer residence times than their Br analogues (blue). Right: Comparison of cell proliferation and antitumor efficacy. The CF₃-substituted compounds show superior efficacy in tumor growth inhibition relative to the Br analogs despite their similar EC₅₀ values. Adapted from Ayaz, P. et al. (2016).

References

1. Ayaz, P. et al. (2016) ACS Chem. Biol. 11, 1710–1719.

It is now understood that drug safety is difficult to achieve by solely considering affinity. Indeed, many reports link selectivity to the kinetic profile of the drugs.

Find out more by reading the appropriate case study:

Affinity doesn’t tell the whole story of MET inhibitor Foretinib

Systemic exposure as a driver of pharmacologic effects provides limited value in guiding compound optimization, selection, and advancement when PK/PD disconnects are observed within a chemical series.  Binding kinetics can help in this scenario not only guiding compound design but also predicting target occupancy and drug dose.

Client feedback and several studies^1-8 suggest that the pharmacokinetic/pharmacodynamic models that integrate binding kinetics, instead of just affinity, better predict:

• Target engagement.
• Drug dose.
• Treatment schedule.
• Real selectivity and potential toxicities.

Simulations of the time course of in vivo target occupancy (TO) by Dasatinib show that conventional PK/PD approaches predict lower efficacy and a higher dose than is actually needed. Affinity based models also fail to predict the real selectivity and consequently, potential toxicities.

This effect of binding kinetics on drug PK has been demonstrated in mice. The comparison of the PK profile of inhibitors with similar K_d but different residence time between the WT and epoxide hydrolase (EH) knockout mice, reveals that drugs with longer residence time bind to EH longer, protecting them from being metabolized or eliminated from the body.

References

1. Georgi V. et al (2018) J Am Chem Soc. 140(46):15774–15782.
2. Daryaee F. et al (2017) Chem Sci. 8(5): 3434–3443.
3. Walkup G.K. et.al (2015) Nat Chem Biol. 11(6):416–423.
4. Lee K.S.S. et al. (2019) ACS Central Sci. 5(9):1614–1624.
5. De Witte W.E.A. (2018) J Pharmacokinet Pharmacodyn. 45(4):621-635.
6. Ramsey J.S. et al (2011) Br J Pharmacol. 164(3):992-1007.
7. Hong Y. et al. (2008) Clin Pharmacokinet. 47(2):129-37.
8. Yassen A. et al (2005) J Pharmacol Exp Ther. 313(3):1136-49.

Understanding binding kinetics early in drug discovery gives access to a more diverse chemical space, better defined biology and therefore more scope for intellectual property.