2.1 ‘Molecular mimics’ a s approaches for rational inhibitor design
Step 4. Finally loss of a proton gives the ester product and regenerates the acid catalyst.
5. PHARMACOLOGICAL RESULTS AND DISCUSSION
5.4 Summary of structure-activity relationships
Our novel substitutions on butabindide permitted us to draw the following conclusions as important for more potent inhibitors:
• The CF3 appears to confer high affinity at the P* site.
• With respect to the effect of the substituents on the aromatic ring both electron- releasing and electron-withdrawing groups were found to be favourable or tolerable if not too large.
• Derivatization of the aromatic moiety was quite tolerant of variations in lipophilicity. • The aromatic ring must be 4- or 5- substituted only.
• Removing the substituent or di-substitution resulted in a decrease in activity. • SAR at P^ was much more restricted: ethyl was optimum.
• The stereochemistry of the side-group at P‘ is important: only the S isomer was found to have an inhibitory effect.
UCL 1397-H K; = T.OnM
Chloro substituent in position 5
Exists as 2 diastereoisomers (the S-diastereomer being
NH
the more active)
Trifluoro substituent NH Ethyl chain UCL 2000 Ky = 0.4nM optimal
It is self evident that any account of drug discovery must be incomplete and certainly only a small portion of the total studies made have been described in this chapter. Many avenues examined during structure-activity analysis turned out to be ineffective and since, in the main, these are not mentioned here, the net effect may be to make the work appear more rational and more perceptive than is warranted.
Pharmacological Results and Discussion 154
An important criterion for the usefulness of UCL 2000 is its biological activity after oral administration. In other words, the efficacy of UCL 2000 is not determined by its pharmacodynamic characteristics alone. It also depends to a large degree on its pharmacokinetic properties since pharmacokinetic processes control the rate and the extent to which UCL 2000 can reach its site of action (CCKa receptors in the brain or stomach). The obvious implications of drug metabolism are an effect on half-life in vivo and the production of toxic metabolic products.
5.4.1 Inhibition o f T PPII activity in vivo: effect o f UCL 2000
Results gained with oral administration of UCL 2000 are compared to those attained with injection of butabindide UCL 1397. This is best done by comparing the corresponding areas under the time-effect curves of both compounds in various parts of the body (Graphs
a-g).
100,
CL
II
50,INHIBITION O F TPPII ACTIVITY IN VIVO
effe c t of UCL1397( I m g /K g i,v.)
A U C:712 3 rai.i
K
(a)
60 —r—120 180 240 T im e (fwin) lO O , o.11 -
X cINHIBITION O F TPF1I ACTjVTTY IN VIVO e ffe c t o f U CL1307 (1m g,'K g i.v } A U C :3560 blocd 60 120 180 24 0 300 360 T im e (m in)
(b)
INHIBITION O F TPPII ACTIVITY IN VIVO a ffe c t o f U C L1397 (1 m $ /K g i.v.) lOO, a §;> 50. I SJZ liver Tim e (m in) (C)
INHSITION OF SPft ACTIVITY IK VIVO effect of UCL2000 (0 5rng.*kg po)
100-1 £ i E so- / r S ;
(d)
«0 AÜC: 16240 8 1 0 0 0 120 1 8 0 2 4 0 T i K E ( f r t n ) 3 0 0 3 6 0 (e)IN rtaTIO N O FSPU A C TM TY iW VÏVO e f fe c t o f UCL 2ÛÙ0 (O .S m g /K g p o ) 6 0 3W too £ ? so4 9 I S (f) i n h ib it io n OF SPtACTTVTTY IN VIVO e ffe c t o f UCL2O00 (0 .5 m g lk g p o ) • UVER A U C ; 2 7 6 S O —r - 00 120 1 0 0 2 4 0 •nWEfmim 3 0 0 3 6 0
iNHBinoKOF SP1I ACTivrry IN v iv o
e f fe c t o f U CL2000 (0.5 m g * g p o )
cuoo
(g)
ALC - 24300
5 0 1 2 0 1 8 0 7 4 0 3 0 0 3 * 0
From such experiments the following was inferred:
There is a certain latent period before the action of UCL 2000 is initiated in the liver, blood and brain since the drug must be absorbed through the mucous membrane of the gastrointestinal tract and into the bloodstream before it reaches every organ in the body. This latent period is commonly seen in humans when (drug) administration is by a route remote from the intended site of action. The intravenous route taken by butabindide however, bypasses the natural barriers of the body to absorption, and therapeutic blood and organ levels are reached almost instantaneously.
Absorption of UCL 2000 is fairly rapid with a peak plasma, liver and duodenum effect reached within 30 min. However due to slow penetration of the drug into and out of
Pharmacological Results and Discussion_____________________________________________ 156
brain, the concentration in the brain rises even after the plasma concentration starts to fall
• UCL 2000 has a high affinity in the liver, and bloodstream but does not to a considerable extent penetrate the brain. TPP II activity inhibition was, on average 15% in brain, 90% in liver, 75% in blood and duodenum.
• Although UCL 2000 is rapidly concentrated in the liver it is not affected by a first-pass liver metabolism or presystemic biotransformation, sufficient blood levels are obtained and prolonged action (bioavailability) when administered orally will lead to good compliance.
• Due to enhanced metabolic stability drug response is elicited at lower doses. UCL 2000 p.o. induced a dose dependent inhibition of food intake with an ED50 of 0.02 mg/kg and a maximal effect of 25%.
In many ways the metabolic properties of UCL 2000 looks to have turned out to be very compatible with the needs and demands of an oral therapeutic.
5.4.2 Binding o f UCL 2000 on proteins
Free diffusion of a molecule to its target site is limited not only by membrane barriers but also by its interaction with endogenous macromolecules other than its binding site. Significant binding to such macromolecules, such as plasma proteins, can influence the overall pharmokinetic fate.
• A similar binding of UCL 2000 (around 50%) was observed as evaluated by the Ki determination in the absence and presence of human serum albumine (HSA) 40g/l (HSA accounts for about 60% of all plasma proteins). Levels of the free drug in the blood supply (which determines the pharmacologic effect) remained unchanged (not shown).
Conclusion. In this chapter we have described the ideas leading to the synthesis of UCL 2000 and some of the approaches we adopted towards the discovery of novel TPP II inhibitors. UCL 2000 is the best compound so far. 18 times more potent in vitro than butabindide and sufficiently active to be given orally, it has been selected for further pharmacological evaluation and shows promise as a much needed, effective therapy for obesity.