Screening& Bioassay of drugs

Transcription

1

2 Screening& Bioassay of drugs Screening?? (qualitative) Sets of procedures performed in experimental animal to detect any pharmacological activity the new compound may have. Bioassay?? (quantitative) Quantitative determination of potency & toxicity by means of dose response relationship. Types of screening 1- Blind screening 2- Simple screening 3- Programmed screening To discover if the new compound has any useful pharmacological activity To find if the new compound has particular pharmacological activity e.g. test for BGL useful for the screening of hypoglycemic activity. To perform full investigation of the most important pharmacological activity of the new drug e.g. mechanism, side effects, drug interactions, etc.

3 Blind Screening 1) Neuropharmacological test 2) Cardiovascular test 3) Cat nictitating membrane (CNM) test 4) Isolated organ preparations e.g. Isolated guinea pig ileum

4 1. Neuropharmacological Test (NPT) The effect of the drug on conscious animal behaviors & Estimation of LD50 Goals of the NPT? are to determine i. The central action of the drug e.g. Sedatives, hypnotics, tranquilizers, muscle relaxants psychomotor stimulants. ii. The autonomic action of the drug e.g. Acetylcholine (Ach) like drugs, sympathomimetic, ganglion blockers, vasodilators, etc. Importance or uses of NPT: 1. It gives an idea about the pharmacological group of the new compound. 2. It gives an approximate lethal dose 50 (LD50)?

5 Type of animals used: Mice for the following reasons: 1. They are mammals, small (i.e. easy to handle) and economic. 2. They are sensitive to most drugs and can be bred to obtain pure and uniform strains. 3. Their life span is long enough Dose: Since the effective dose is not completely known, more than one dose level are used (10, 30, 100, 300 and 1000 mg/kg). Route of administration: Intrapretonial (I.P) to avoid the possibility of poor absorption from GIT.

6 Scoring system: Arbitrary, scaled from 0-8 Normal behavior before drug administration take score 4 (Reference score) Subnormal response ( in normal behavior after drug adm.) < 4 Supernormal response( normal behavior after drug adm. > 4 Abnormal response after drug administration take score 0 e.g. convulsion NB: Abnormal behavior must be excluded before the administration of the unknown I.a. For CNS observations the normal signs include: 1-Consciousness (awareness): (4) CNS stimulation ordepression 2-Response to manipulation: (4) Subnormal responses tranquilization, central depression, sk.m. relaxation, paralysis or anesthesia. 3-Motor activity and spontaneous activity: (4) Subnormal responses depression ofans, ganglia or NMJ. 4- Pain response: (4) Subnormal responses Analgesia, CNS depression or sedation. 5- Touch response: (4). Subnormal responses anesthetic activity.

7 I.b. For CNS observations the abnormal signs include: Any abnormal sign before drug adm must be excluded 1. Stereotypy (0):frequent repetition of a purposeless movement a) Morphine (circular movement). b) Amphetamine (searching movements of the head). 2. Straub (erection) tail (0). tremors, convulsions and opisthotonus? (0). central excitation restlessness, irritability and fearfulness: (0). 3-Motor incoordination ( (0). e.g. staggering gait & Loss of righting reflex CNS depression. Muscle relaxation. Anesthesia. Q-Opisthotonus: severe hyperextension of head, neck and spinal column forming arched position. It caused by spasm of trunk muscles along the spinal cord What are the signs indicating central excitation.autonomic response?.

8 II- For ANS observations: -Normal sign -Abnormal sign: Pupil size :(4). Exophthalmoses:(0). Respiratory rate: (4). Piloerection: (0). Heart rate: (4). Secretions: (0). Writhing: (0). it indicates autonomic acting drug

9 2. Cardiovascular test (CVT) This test investigates the effect of the new drug on the blood pressure (B.P.) of anesthetized cat Procedure: 1. Cat is anaesthetized with chloralose (chloral hydrate + 5% glucose). chloralose is preferred?? as it produces a steady state of anesthesia for long time & does not affect autonomic response or B.P. 2. The femoral artery is cannulated for recording BP 3. The femoral vein is cannulated for I.V. injection of the drug 4. The trachea is cannulated for artificial respiration 5. Carotid arteries are exposed to induce carotid occlusion (C.O) 6. vagus nerves cut bilaterally, attach an electrode so electrically stimulated (ES)

10 Tetrad system 1- ES of the vagus for 2 sec (VS) Indirect action..stimulate parasymp VD, BP B.P.? VS 2- Ach injection (5 μg/kg) (Ach) direct peripheral muscarinic action VD, bradycardia 3- Carotid occlusion for 45 sec (CO) Indirect action 4- Injection of 5 μ g/kg Ep (EP or NE) B.P.? direct action stimulates β1 receptor HR, Stimulates α1 receptor of arterioles VC & PR B.P.??? B.P.? Ach CO EP or NE VS Ach CO NE

11 Indirect action BP in carotid sinus so inhibitory impulses from Baroreceptor reflex stimulation of VMC & CAC PR, HR &BP 11

12 6-Inject the unkn compound (5 μg/kg) & observes the change in BP Drug s effect 1. Brief fall in B.P. Graph drug Drug may be a. Parasympathomimetic b. Vasodilator c. cardiodepressant 2. Brief rise of B.P. 3. Prolonged fall in B.P. 4. Prolonged rise in B.P.. Delayed (after45 sec.) fall in B.P. dr dr dr dr a. Sympathomimetic b. Vasoconstrictor a. Anticholinestrase b. Ganglion blocker c. tranqilizer e.g. CPZ a. MAOI e.g. phenelzine a. Histamine liberator

13 7- Perform tetrad then injected 5 μg/kg of test substance and repeat the tetrad, observe the changes in the response to the 4 tests of tetrad Drug s effect Graph Drug may be 1. effect of VS & Ach a. Parasympatholytic dr VS Ach C.O 2. Potentiate VS & Ach a. Anticholinestrase dr+ + E 3. effect of CO &E a. Sympatholytic dr 4. Potentiate CO & E dr + + a. MAOI b. Cocaine 5. Block VS & CO a. Ganglionic blocker dr

14 Drug s effect 6. Block CO & potentiate E dr Graph + VS Ach C.O E Drug may be a. Adrenergic neuron blocker If the unkn was ANB, what will be its effect on tetrad?

15 3. Cat nictitating membrane Test An additional (3 rd ) eyelid that Composed of smooth muscle, Innervated by sympathetic nerve fibers, Contains α adrenergic receptors and Responds by contraction. It moves horizontally and is responsible for light regulating effect It is used to detect & differentiate between 1. Sympatholytics (ANBs, Ganglionic blockers & alpha blockers) 2. Sympathomimetics (EP or NE) 3. Anticholinesterase

16 Procedure: 1. The cat is anesthetized and the membrane is attached to a lever for recording the contractions 2. An electrode is attached to the preganglionic nerve fiber and another electrode for the postganglionic nerve fiber 3. Three control tests are performed (Triad system) ES of preganglionic nerve fiber ES of postganglionic nerve fiber Exog. NE injection. 4- The drug is injected and the triad system is repeated

17 Possible conclusions response of the membrane to symp. depressants Cervical symp. nerve Nictitating membrane the DRUG may be Pre post R ES ES NE Pre post R ES ES NE Pre post R ES ES NE Pre post R ES ES NE Pre post R ES ES NE Pre post R ES ES NE Pre post NE Normal contraction Ganglionic blocker Adrenergic neuron blocker Alph-1 receptor blocker Sympathomimetic Anticholinesterase

18 4. Isolated organ preparations In vitro prep? Examples: Guinea pig ileum Rat uterus Rabbit intestine Frog rectus abdominis muscle Guinea pig tracheal chain

19 Advantages: 1. No interference by central innervations, circulatory hormones, changes in blood flow etc. 2. Several preparations can be obtained from a singe animal 3. Small amount of tested material is required. 4. Drug effect is directly tested without the factors of absorption, distribution, metabolism and excretion. Disadvantages: 1.Drug effect may be modified by ph, composition of physiological solution, temp & oxygen supply. 2. The effect observed in vitro may not be the same as that observed in vivo 19

20 The guinea pig ileum It is an example of smooth muscle preparation It is the last part of small intestine Advantages of guinea pig ileum over the rabbit intestine 1- It is easy to set up, 2- It resists trauma during handling 3- It has larger contractions than jejunum 4- It shows little or no myogenic contraction 5- It contains many receptors e.g. Adrenergic, muscarinic, nicotinic, histamine & 5HT receptors Uses: to detect whether the drug is an agonist or antagonist for ACh, nicotine, histamine or serotonin receptors NB: rabbit intestine is not used instead of guinea pig ileum for bioassay of histamine?? as it contains histaminase that breaks histamine so large dose is required GP. Ileum has many advantages

21 Possible conclusions If the unkn is agonist and its effect is blocked by Graph The DRUG may be 1-Hexamethonium (GB) 2-Atropine [M ] unkn HXM unkn unkn Atro unkn Ganglionic stimulant e.g. dil nicotine parasympathomimetic (Ach) 3-Mepyramine (H1 ) unkn mpyr unkn H1 agonist [histamine] 4-bromo LSD [Anti-5HT] Lysergic acid diethylamide unkn LSD unkn 5HT agonist (serotonin) If the effect of unkn not blocked by 1-4 Unk 1 unk 2 unk 3 unk 4 unk Direct smooth muscle stimulant (BaCl 2 )

22 If the unkn causes no contraction (antagonist) but inhibit response to e.g. Ach it is atropine, etc. histamine..it is antihistamine 5HT.it is antiserotonin dil nicotine.it is GB unkn Ach Unkn +Ach Ach, nic, hist, 5HT.sympthomimetic or DSMR. Differentiated by addition of α, β blockers then..direct ES of!ileum Contraction sympathomimetic. No contraction DSMR. (morphine like drug)

23