title: Unexpected relationship between plasma protein binding and the pharmacodynamics of 2-NAP, a CCK1-receptor antagonist.
abstract: WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT?: * Two chemically diverse CCK1 receptor antagonists have been shown clinically to inhibit CCK-evoked contraction of human gallbladder [2, 3]. These studies have not examined the relationship between plasma concentration and effect, the latter usually considered to be predictive from the free drug concentration [8]. * We wanted to examine our novel CCK1 receptor antagonist in this validated model and also to explore its PK-PD relationship.
  
  
  CONCLUSIONS
  * 2-NAP inhibited CCK-evoked human gallbladder contraction in vivo but at a plasma free concentration that was, in theory, too low to have achieved adequate CCK1 receptor occupancy. * The study serves as a caveat to the assumption that free plasma concentration can be used to predict pharmacological effect.
  
  
  OBJECTIVE
  To study the pharmacokinetics and pharmacodynamics of 2-NAP (2-naphthalenesulfonyl-L-aspartyl-(2-phenethyl)amide), a selective CCK1 receptor antagonist in healthy volunteers.
  
  
  METHODS
  2-NAP was given to 12 healthy male volunteers in an ascending dose, safety and PK phase 1a study by 1 h i.v. infusion (0.6-9.6 mg kg(-1) h(-1)). A further 12 healthy male volunteers received i.v. CCK-8S (6.25 pmol kg(-1) h(-1)) to produce gallbladder contraction, measured by ultrasound recordings of gallbladder volume, and the effect of concurrent i.v. 2-NAP administration was studied. Plasma protein binding in vitro and ex vivo was measured by ultrafiltration and by equilibrium dialysis.
  
  
  RESULTS
  2-NAP was generally well tolerated, displayed linear pharmacokinetics and a very high degree of plasma protein binding (99.9%). A 105 min i.v. CCK-8S infusion induced a reduction in gallbladder volume of 14.9 (+/-7.0) ml during placebo co-infusion and this was reduced to 2.4 (+/-5.9) ml when 2-NAP was co-infused with CCK-8S (P = 0.00024, paired t-test, mean change 12.5 ml; 95% CI For mean 7.4, 18.3 ml). This extent of inhibition was consistent with a 2-NAP total plasma concentration of 36 microm, but when protein binding corrections were made, the 'free concentration' of 2-NAP was only 0.04 microm, a value much less than the average equilibrium dissociation constant of 2-NAP for human CCK1 receptors ( approximately 0.7 microm).
  
  
  CONCLUSIONS
  The pharmacological effect of a drug is usually considered to be determined by its free concentration. However, the complete inhibition of CCK-8S-evoked gallbladder contraction by a free plasma concentration of 0.04 microm 2-NAP was much greater than would have been predicted from simple drug-receptor occupancy theory and cautions against the general use of free concentration of drug for predicting pharmacological effect.
authors.affiliation: James Black Foundation, 68 Half Moon Lane, London, UK.
authors.forename: V P
authors.initials: VP
authors.lastname: Gerskowitch
authors.forename: J
authors.initials: J
authors.lastname: Hodge
authors.forename: R A D
authors.initials: RA
authors.lastname: Hull
authors.forename: N P
authors.initials: NP
authors.lastname: Shankley
authors.forename: S B
authors.initials: SB
authors.lastname: Kalindjian
authors.forename: J
authors.initials: J
authors.lastname: McEwen
authors.forename: J W
authors.initials: JW
authors.lastname: Black
mesh_terms: D000293:Adolescent; D000328:Adult; D001224:Aspartic Acid / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000494:pharmacology; D001798:Blood Proteins / Q000378:metabolism*; D018592:Cross-Over Studies; D005704:Gallbladder / Q000000981:diagnostic imaging; D016473:Gallbladder Emptying / Q000187:drug effects; D006801:Humans; D008297:Male; D009282:Naphthalenesulfonates / Q000493:pharmacokinetics* / Q000494:pharmacology; D011485:Protein Binding; D043886:Receptor, Cholecystokinin A / Q000037:antagonists & inhibitors*; D011949:Receptors, Cholecystokinin / Q000819:agonists; D012844:Sincalide / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000494:pharmacology; D016037:Single-Blind Method; D014463:Ultrasonography
chemical_list: C090019:8-sulfocholecystokinin octapeptide; D001798:Blood Proteins; D009282:Naphthalenesulfonates; D043886:Receptor, Cholecystokinin A; D011949:Receptors, Cholecystokinin; C080661:N-(2-naphthalenesulfonyl)aspartyl-(2-phenethyl)amide; D001224:Aspartic Acid; D012844:Sincalide