Potential for delta opioid receptor

agonists as analgesics in chronic

pain therapy

David Kendall & Bengt von Mentzer; PharmNovo AB/UK Alex Conibear & Eamonn Kelly, University of Bristol Junaid Asghar, Gomal University, Pakistan

Problems with conventional opioid agonist analgesics:

• Acutely toxic in high dose (respiratory depression). • Peripheral side effects (constipation, itching). • Rapid receptor desensitization leading to tolerance and dose

escalation. • Euphoric with abuse potential/dependence. • Only recommended as third line chronic pain treatment.

Most, if not all, problems are μ opioid receptor-mediated

Could δ opioid receptor agonists provide a beneficial alternative in chronic pain??

Single gene product but differential agonist effects suggest possible post-translational modification; DOR1 and DOR2 subtypes suggested.

Delta opioid receptor (DOR) structure

Structures of representative non-peptide δ-opioid receptor

agonists.

JNJ-20788560 PN6047

Delta opioid receptor (DOR) agonists & chronic pain

• DOR agonists weakly modulate acute nociception, but a variety of delta opioid agonists are effective under experimental conditions involving persistent pain (also in chronic morphine-treated animals).

• Conditional sensory nerve DOR knockout mice (NaV1.8) show only subtle, modifications of pain thresholds in acute pain models but display increased neuropathic and inflammatory pain.

Conditional KO of DOR in peripheral sensory nerves enhances neuropathic pain (sciatic nerve ligation)

Nav 1.8 Oprd1-

Acute nociceptive stimuli including tail immersion, tail flick, hot plate, plantar Hargreaves, Von Frey filaments, tail pressure and acetic acid writhing unchanged in KOs

Gaveriaux-Ruff et al., 2011

KOs are nortriptyline-insensitive so DORs may be downstream effectors of TCA’s chronic pain effects.

WT

Analgesic effects of SNC80 in neuropathic pain model (SNL) abolished in sensory nerve conditional DOR KOs

Gaveriaux-Ruff et al., 2011

Nav 1.8 DOR KO

WT

Delta opioid receptors regulate cutaneous

mechanosensation (touch)

• DORs are expressed by myelinated low-threshold

mechanoreceptors that encode touch.

• DOR-expressing myelinated dorsal root ganglion neurons innervate

mechanosensory organs in the skin (Merkel cells, hair follicles,

Meissner corpuscles).

• DOR activation depresses neurotransmitter release at the central

terminals of mechanoreceptors.

• Well established that DOR agonists inhibit mechanical

nociception; so there’s a good case for using DOR agonists to

treat mechanical allodynia in chronic pain states.

DOR and MOR Expression in Cutaneous Sensory Receptors in Skin

DOR-expressing DRGs are sensitive to innocuous mechanical stimuli, such as light touch, while the MOR expressers are stimulated by noxious stimuli (Bardoni et al. 2014).

Widespread, largely segregated distribution of DOR and MOR in spinal cord.

DOR in somatostatin-positive dorsal horn interneurons controls mechanical pain and but not heat pain (Wang et al., 2018).

Concerns about DOR agonist- induced seizures have inhibited pharmaceutical development

SNC 80-induced seizures are abolished in total DOR KO and in forebrain GABAergic neurone KOs

Chung et al., 2015

Other DOR agonists (e.g. ARM-390 and ADL-5859) showed no convulsant properties.

WT Pan-KO

Brain KO

PN6047 does not affect pentylenetetrazol (PTZ)-induced seizures in mice

SNL-induced allodynia P T Z in d u c e d s e iz u re s - H in d lim b to n u s

Veh

icle

(n

=10)

PN

6047 (

n=10)

0

5 0

1 0 0

1 5 0

S ta t is t ic a l a n a ly s is , u n p a ir e d t - te s t u s in g P r is m

N o s ig n i f ic a n t d i f fe ra n c e w h e n c o m p a re d to v e h ic le t re a te d m ic e .

Do

se

(m

g/k

g)

No respiratory depression with DOR agonist PN6047

No effect of PN6047 on tail flick

Rob Hill & Graeme Henderson, Univ. Bristol

Abuse potential

JNJ-20788560 was not self-administered and had no reinforcing effects in rhesus monkeys (Codd et al., 2009). No withdrawal syndrome after repeated treatment. SNC80 did not produce reinforcement in monkeys trained to self administer heroin (Stevenson et al.,2005). No reinforcement with AZD2327 in cocaine self-administering primates (Hudzik et al., 2014).

DOR agonist bias towards G protein signalling might reduce unwanted properties

Tolerance

Agonist selective recruitment of arrestin isoforms differentially modify DOR function (Pradhan et al.,2016;

Dripps et al.,2017)

High internalizing agonists (e.g. SNC80) preferentially recruit arrestin 2. Arrestin 2 KOs show enhanced potency in inhibiting mechanical hyperalgesia and inducing convulsions, decreased analgesic tolerance.

Low internalizing agonists (e.g. ARM390 and JNJ20788560) preferentially recruit arrestin 3; But arrestin 3 KOs exhibit acute tolerance to these agonists (never seen in wild type).

All DOR agonists tested fully engage Gi protein signalling; but some are only partial arrestin recruiters

δ Opioid Receptor Internalization

Co

ntr

ol

Fo

rsko

lin

(1M

)

Fo

rsk+ S

NC

80 (

1M

)

Fo

rsk+ D

AD

LE

(1M

)

Fo

rsk+P

N6047(1M

)

Fo

rsk+P

N6047+N

al

0

2 0

4 0

6 0

8 0

1 0 0

cA

MP

(a

rb

itra

ry

un

its

) C o n tro l

F o rs k o lin (1 M )

F o rs k + S N C 8 0 (1 M )

F o rs k + D A D L E (1 M )

F o rs k + P N 6 0 4 7 (1 M )

F o rs k + P N 6 0 4 7 + N a ltrin d o le

***

Signalling desensitization in vitro cAMP inhibition due to PN6047 does not does desensitize after 48hrs exposure to a supramaximally effective concentration; no remaining effects of DADLE or SNC80.

Analgesic Tolerance

Conventional opioid agonists cause rapid desensitization, leading to analgesic tolerance. Whilst a single administration of SNC80 is effective in reducing inflammatory pain (CFA), a second is without effect (Pradhan et al., 2009); accompanied by DOR internalization and G protein uncoupling. In contrast, ARM390 retained analgesia over 5 days of treatment without significant DOR phosphorylation or internalization.

Daily administration of PN6047 (3mg/kg p.o.) in SNL neuropathic pain model.

Are differences in arrestin recruitment directly related to the pharmacological profiles of δ opioid receptor agonists?

Other potential indications for DOR agonists?

• Anxiety /depression; efficacy in acute (forced swim) and chronic (olfactory bulbectomy) models. Anxiolytic trends in clinical trial of AZ2327 in anxious depression.

• Migraine • Neuroprotection • Cardioprotection • Cough

Conclusions

• δ opioid receptor agonists have preclinical efficacy in models of chronic pain, particularly those with a mechanical hypersensitivity component, without unwanted μ receptor-like side effects.

• Seizure induction is not a drug class property.

• Could DOR agonist drugs replace conventional opioids?

Dr Junaid Asghar

Dr Alex Conibear

People who did most of the work