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Screening methods for analgesics

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Screening of analgesics
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Screening methods for analgesics

  1. 1. SCREENING METHODS FOR ANALGESICS DR. AMIT D. SHARMA Third year resident, Department of pharmacology SBKS MI & RC. Date : 09/05/2015
  2. 2. Contents  Definitions  Central analgesics : agents, mechanism, in vivo methods  Peripheral analgesics : agents, mechanism, in vivo methods
  3. 3. Definitions  Pain- an unpleasant sensory or emotional experience associated with actual or potential tissue damage, or described in terms of such damage (WHO).  Features of inflammation Pain (dolor) rubor (redness) tumor (swelling) calor (heat) functio laesa (loss of function).
  4. 4.  Analgesics – the drugs which possess significant pain- relieving properties by acting in the central nervous system or on peripheral pain receptors without significantly affecting consciousness.  Two groups : I. Narcotic/opioid /morphine like analgesics(central) II. Non-narcotic/ non steroidal anti-inflammatory- analgesic-antipyretic agents(peripheral)
  6. 6. Classification  Morphine analogues: I. Agonists – morphine(prototype), diamorphine(heroin), codeine, pholcodeine, levorphanol II. Partial agonists – nalorphine, levallorphan  Synthetic derivatives unrelated to morphine : I. Agonists – Pethidine (meperidine), fentanyl, sufentanil, alfentanil, remifentanil, methadone, oxycodone II. Partial agonists – pentazocine, cyclazocine, buprenorphine, nalbuphine, butorphanol III. Mu receptor agonist with other mode of action – tramadol
  8. 8. Haffner’s tail clip method, 1929  The test compounds are administered subcutaneously to fed mice or orally to fasted animals.  Then an artery clip is applied to the root of the tail (approximately 1 cm from the body) of mice.  The reaction time is noted.  Response : biting the clip or the tail near the location of the clip.
  9. 9. Hot plate method  Woolfe and Mac Donald (1944)  Mice are put on hot plate(temp 55° to 56 °C) and then latency is noted following administration of test drug.  Response – jumping or paw licking
  10. 10. Tail immersion method  Restraining the young female wistar rat, lower 5 cm portion of the tail is immersed in a cup of freshly filled water of exactly 55°C.  The reaction time is determined before and periodically after either oral or subcutaneous administration of the test substance.  The cut off time of the immersion is 15 seconds.  Response – withdrawal of tail or attempt to escape.
  11. 11. Grid shock test  Blake et al(1963)  Central and peripheral analgesics  Stimulus is given in the form of square wave pulses to male mice. The output of the stimulator is connected to the wires of grid with fixed resistance , which is kept parallel to oscilloscope.  Pain thresholds are determined in each individual mouse twice before and after administration of the test drug.  Response – on oscilloscope, a startling reaction, increase locomotion or attempt to jump.
  12. 12. Electrical stimulation of the tail  Burn et al, 1950  Central and peripheral analgesics  First test compound is given and then rectangular wave pulses from a constant voltage stimulator are applied to the alligator clips attached to the tail of the mice.  Response – spinal reflexes, complete vocalization or behavior changes(escape, aggression). The normal response time range of the stimuli is 3–4 sec.
  13. 13. Tooth pulp stimulation  Kohl and Reffert (1938)  Opioid agonists, pyrazolone derivatives  After anaesthetizing the rabbit with thiopental, current is applied through the electrodes passed in the dental drill holes exposed to pulp chambers.  Pain thresholds are determined in each individual rabbit twice before and after administration of the test drug.  Response - licking, biting, chewing or head flick.
  14. 14. Formalin test in rats  Dubuisson and Dennis,1977  Male wistar rats are administered 0.05 ml of 10% formalin in dorsum of the front paw along with test drug subcutaneously or orally.  Response - elevation or favoring of the paw or excessive licking and biting of the paw.
  16. 16. Classification  Nonselective COX inhibitors (traditional NSAIDs) I. Salicylates – aspirin II. Propionic acid derivatives – ibuprofen, naproxen III. Fenamide – mephenamic acid IV. Enolic acid derivatives – piroxicam V. Acetic acid derivatives – ketorolac, indomethacin VI. Pyrazolone derivatives – phenylbutazone
  17. 17.  Preferential COX-2 inhibitors - nimesulide, diclofenac, aceclofenac, etodolac.  Selective COX-2 inhibitors – celecoxib, etoricoxib.  Analgesics-antipyretics with poor antiinflammatory action- I. Paraaminophenol derivative – paracetamol(acetaminophen). II. Pyrazolone derivatives – metamizol(dipyrone), prophenazone. III. Benzoxazocine derivative – nefopam.
  19. 19. Writhing tests  Mice are administered the test drug before intraperitoneal injection of irritating agent like phenylquinone or acetic acid.  Pain sensation in acetic acid induced writhing paradigm is elicited by producing localized inflammatory response due to release of free arachidonic acid from tissue phospholipids via cyclo-oxygenase (COX), and producing prostaglandin specifically PGE2 and PGF2α, the level of lipoxygenase products may also increases in peritoneal fluids.
  20. 20.  Response – writhe (stretching of the abdomen with simultaneous stretching of at least one hind limb)  Aconitine can also be used instead of above drugs.  Rats – 4% sodium chloride.
  21. 21. Pain in inflamed tissue (RANDALL-SELITTO TEST)  Principle - inflammation increases the sensitivity to pain and that this sensitivity is susceptible to modification by analgesics.  Male wistar rats are given the test drug orally, subcutaneously or intraperitoneally.  Inflammation is induced by subcutaneous injection of Brewer’s yeast into the plantar surface of the left hind paw of the rat .Then pressure is applied through a tip to the plantar surface of the rat’s foot at a constant rate by a special apparatus to the point at which the animal struggles, squeals or attempts to bite.
  22. 22. Mechanical visceral pain model in the rat  Coburn et al 1989,deLeo et al 1989.  First test drug is administered into male Sprague Dawley rats and anaesthetized with nitrous oxide and halothane.  One piece balloon catheter is passed through gastrostomy tube into first part of duodenum.  Inflation of balloon distension of duodenum writhing.
  23. 23. Behavioural responses score 0 : Normal behaviour defined as exploration, escape attempts and resting 1 : Slightly modified behaviour defined as cessation of exploration, focusing, wet-dog shake, excessive facial grooming, teeth chattering and deep breathing 2 : Mildly to moderately modified behaviour defined as retching-like activity, hunching, abdominal grooming or nipping and immobility of the hind limbs (disappears with removal of the stimulus).
  24. 24. 3 : Severely modified behaviour defined as stretching of the hind limbs, arching and dorsoflection of the hind paws. 4 : Intensive visceromotor activity defined as repetitive stretching of the body, extension of the hind limbs, and pelvis, frequent rotating sideward , i.e., writhing.
  25. 25. References  H. Gerhard Vogel et al. Drug discovery and evaluation : Pharmacological assays, third edition.  K D Tripathi. Essentials of medical pharmacology, seventh edition.  H L Sharma, K K Sharma. Principles of pharmacology, second edition.  Harsh Mohan. Textbook of pathology, sixth edition.
  26. 26. THANK YOU

Editor's Notes

  • 0.02% conc. In 1% suspension of carboxymethylcellulose (0.25 ml)
  • 0.1 ml of a 20% suspension
  • 0.1, 0.25, 1 and 10 mg/kg indomethacin i.p.