New and novel uses for paracetamol

(1)

Dr Samir S. Ayoub

New and novel uses for paracetamol

(2)

Paracetamol

• Analgesic and fever drug with weak

anti-inflammatory effects

• Used clinically for over a century

• Mechanism of pharmacological action still not fully

established

• Cyclooxygenase (COX) enzymes are involved in

production of prostaglandins, which mediate

inflammation, pain and fever

(3)

Mode of action of NSAIDs

NSAIDs work by inhibition of prostaglandin biosynthesis

by inhibition of COX activity

Inhibition (%)

Indomethacin

Aspirin

Salicylic acid

Log concentration (µg/ml) (Vane, Nature 1971)

100 80 60 40 20 0

0.1 1.0 10 100 1000

(4)

Inhibition of COX activity by paracetamol is

tissue selective - in vitro assay

ID₅₀ µgml-1

Indomethacin 0.06 1.3

Sodium aspirin 6.6 11.0

4 - Acetamidophenol 100.0 14.0

(Flower and Vane, Nature1972)

Synthetase from: Dog spleen Rabbit brain Dog brain

12.5

(5)

(6)

Prostaglandins and NSAIDs

Prostaglandins NSAIDs

Pain Analgesic

Fever Antipyretic

Inflammation Anti-inflammatory

(7)

Hypothesis

(8)

Paracetamol is centrally acting

• Reduced spinal cord released of PGE₂ in response to

capsaicin (Malmberg & Yaksh, 1994)

• Reduced spinal cord released of PGE₂ in the formalin test, but

not the urinary excretion of PGE₂, PGF₂_ & 6-keto-PGF₁_ (Muth Selbach et al 1999)

• Reduced the pain threshold in the flexion reflex to transcutaneous electrical stimulation in man, which is a model of central nociception (Piletta, et al, 1991)

• The antipyretic effect of paracetamol was accompanied by

potent reduction in PGE₂ levels in the cerebrospinal fluid

(9)

COX-2 7 50 15 20 0.35 1.3 0.34 0.84

IC

₅₀

values (µg/ml) of NSAIDs on

COX-2 or COX-1 activity in intact cells

COX-1 0.04 0.3 1 2.7 0.5 2.2 1.2 63 Ratio NSAID Tolmetin Aspirin Ibuprofen

Paracetamol (IC₃₀) Diclofenac Naproxen Celecoxib Rofecoxib COX-2 COX-1 175 166 15 7.4 0.7 0.6 0.3 0.013

(10)

COX-3, a cyclooxygenase-1 variant inhibited by

acetaminophen and other analgesic/antipyretic

drugs: Cloning, structure, and expression

N.V. Chandrasekharan, Hu Dai, K. Lamar Turepu Roos, Nathan K.Evanson, Joshua Tomsik, Terry S. Elton, and Daniel L.Simmons*

Department of Chemistry and Biochemistry, E280 Benson Science Building, Bringham Young University, Provo, UT 84602

Communicated by John Vane, William Harvey Foundation, London, United Kingdom, August 5,2002 (received for review April 17, 2002)

The Discovery of Cyclooxygenase-3

 Splice variant of COX-1

 Most abundantly expressed in cerebral

cortex

 Selectively inhibited by paracetamol

(11)

IC₅₀, (M)

Drug COX-1 COX-2 COX-3

Paracetamol >1000 >1000 460 Aminopyrine* >1000 >1000 688

Antipyrine >1000 >1000 863

Dipyrone 350 >1000 52 Phenacetin >1000 >1000 102 Aspirin 10 >1000 3.1 Diclofenac 0.035 0.041 0.008 Ibuprofen 2.4 5.7 0.24 Indomethacin 0.010 0.66 0.016 Caffeine >1000 >1000 >1000 Thalidomide >1000 >1000 >1000

All assays were carried out in the presence of 30M arachidonic acid. *4-dimethylaminoantipyrine.

(12)

(13)

35.5 36.0 36.5 37.0 37.5 38.0 38.5 39.0 B o d y T e m p ( o C )

0 1 2 3 4 5

0 100 200 Time (h) P G E 2 (p g /w e ll)

The reduction of basal body temperature with 300mg/kg

paracetamol correlates with reduction of brain PGE₂levels

(14)

COX-3 (65KDa) COX-2 (72KDa) COX-1 (72KDa) Cerebral cortex Mid brain Brain stem Cerebellum

(15)

Time-profile to the effect of 300mg/kg paracetamol on the basal body temperature in COX-1 & COX-2 genes knock-out mice

COX-1 gene knockout mice

Paracetamol hypothermia is reduced in COX-1 knockout mice and is retained in COX-2 knockout mice

(16)

The effect of 300mg/kg paracetamol on brain PGE₂ levels after 1hr

Reduction in the hypothermic effect of paracetamol in

COX-1-/- mice is linked to loss of the effect of paracetamol on brain PGE₂

(17)

The effect of SC560 & Celecoxib alone and in combination with paracetamol on the basal body temperature of mice

(18)

1. Conclusions

Reduction of basal body temperature by paracetamol confirms similar findings in humans (Dippel et al, 2003; Tittelboom et al, 1988; Denes et al, 2002)

The dose-dependency & time-profile of the reduction of

temperature confirms that the effect is related to paracetamol.

Correlation of reduction of body temperature and brain PGE₂

confirms that paracetamol targets a central COX enzyme.

The reduction in paracetamol-induced hypothermia and brain

PGE₂ in COX-1 knockout mice confirms that the likely target for

(19)

(20)

Paracetamol is converted to N-arachidonylamine

phenolamine (AM404) in the brain by the action of

fatty acid amide hydrolase (FAAH)

(21)

The tissues involved in the conversion of

paracetamol to AM404

Mallet et al, 2008

CB1 receptor TRPV1 channel

(22)

AM404 has analgesic

(La Rana et al., 2008 & 2006; Mitchell et al., 2007; Costa et al., 2006)

and hypothermic actions

(Rawl el., 2006)

AM404 activates the CB1 receptor-mediated endocannabinoid

(23)

Activation of the brain endocannabionid system

results in hypothermia

Activation of the transient receptor potential

vanilloid-1 (TRPV1) in the brain, also results in

hypothermia

AM404 is able to activate both the endocannabinoid

and TRPV1 systems

Does AM404 mediate the paracetamol-induced

hypothermia through activation of the

(24)

(25)

(26)

(27)

(28)

(29)

(30)

(31)

Conclusions

The paracetamol induced hypothermia is not dependent on the cannabinoid and TRPV1 systems

AM404 does not mediate the paracetamol-induced hypothermia

(32)

Following a stroke, cardiac arrest or neurotrauma brain ischemia leads to significant neuronal cell death leading to long term

disability or death.

Induction of mild, sustained hypothermia is an established method for the acute management of such patients.

Mechanism through which hypothermia protect the brain include, reduction in brain metabolic rate, blockade of excitotoxicity

calcium antagonism, preservation of protein synthesis, a decrease in oedema formation, modulation of the inflammatory response

and modulation of apoptotic cell death.

Most of the damage occurs within the first hour “Golden hour”

(33)

(34)

Therapeutic hypothermia: Current methods

Current methods used to induce therapeutic-hypothermia can not be used in the pre-hospital setting, large in size and expensive.

(35)

Drug-induced hypothermia: fast onset of action

Combinational hypothermia; paracetamol and cannabinoid agonist.

Intravenous formulation

Out-of-hospital use

Induction of hypothermia within minutes

“Inside-out” hypothermia

Easily maintained hypothermia

(36)

The team

Prof. David Baker (Professor of Neuroimmunology, ICMS)

Prof. Gavin Giovannoni

(Consultant Neurologist, Royal London)

Dr Rupert Pearse (Critical Care Consultant, Royal London)