Targeting angiotensin II type 2 receptor pathways to treat neuropathic pain and inflammatory pain

Figures & data

Figure 1. Schematic overview of the renin–angiotensin system.

Figure 2. Chemical structures of the small-molecule angiotensin II type 2 receptor antagonists, viz., EMA200 (also referred to as PD123319), EMA300 (also referred to as PD121981), EMA400 (also referred to as PD126055) and EMA401 ([S]-enantiomer of EMA400).

Table 1. Radioligand binding affinities and relative selectivities at the cloned rat and human AT₂R and AT₁R.

Cloned receptor species	Ligand/AT2R antagonist	AT2R		AT1R		Binding affinity/selectivity AT2R/AT1R
		Mean (nM) (KD or IC50)	SEM	Mean (µM) (KD or IC50)	SEM
Cloned rat receptors	Angiotensin II	1.6	1	0.0149	1.1	9.3
	EMA200	71.7	14.5	210.5	121.5	∼ 3000
	EMA300	46.5	3.3	49.9	10.2	> 1000
	EMA400	75.2	10	2918	1270	> 30,000
	EMA401	39.5	5.2	408	335	> 10,000
	EMA402	804	-	85.4	-	106
Cloned human receptors	Saralasin	0.19	-	0.00048	-	0.4
	EMA401	39	-	n.c.	-	> 10,000 but n.c.
	EMA402	1100	-	59	-	53.6

Adapted with permission from John Wiley and Sons [29].

AT₁R: Angiotensin II type 1 receptor; AT₂R: Angiotensin II type 2 receptor; n.c.: Not calculable; SEM: Standard error of the mean.

Table 2. Analgesic efficacy of selective, small molecule, AT₂R antagonists in rodent models of inflammatory and peripheral neuropathic pain.

Pain type	Rodent model	AT2R antagonist(s) evaluated and dosing regimen	Analgesic effects produced by the AT2R antagonist(s)			Ref.
			EMA200	EMA300	EMA400
Neuropathic	Chronic constriction injury of the sciatic nerve in the rat	Single i.p. bolus doses of EMA200 (1 – 10 mg/kg), EMA300 (1 – 10 mg/kg) and EMA400 (0.003 – 0.03 mg/kg)	✓	✓	✓	[29]
	Dideoxycytidine-induced ATN in the rat	Single i.p. bolus doses of EMA200 at 0.3 – 10 mg/kg	✓	NA	NA	[35]
		Twice-daily doses of EMA300 at 1, 10 and 30 mg/kg i.p. for 3 days	NA	✓	NA	[35]
Inflammatory	FCA-induced inflammatory pain in the rat hindpaw	Acute (10 mg/kg i.p.) and chronic dosing (5 mg/kg/day i.p. for 7 consecutive days) of EMA200	✓	NA	NA	[37]
	FCA-induced monoarthritic pain in the rat knee joint	Single i.p. bolus doses of EMA300 and EMA400 at 0.1 – 10 mg/kg	NA	✓	✓	[36]
Inflammatory and neuropathic	Prostate cancer-induced bone pain in the rat	Single i.v. bolus doses of EMA200 at 0.3 – 10 mg/kg	✓	NA	NA	[15]

AT₂R: Angiotensin II type 2 receptor; ATN: Antiretroviral neuropathy; FCA: Freund’s Compete Adjuvant; i.p.: Intraperitoneal; i.v.: Intravenous; NA: Not assessed.

Figure 3. A schematic diagram summarizing the proposed augmented angiotensin II/AT₂R signaling pathways and their interaction in peripheral neuropathic pain and/or chronic inflammatory pain states.

Table 3. Mean (±SEM) pharmacokinetics and oral bioavailability of small-molecule AT₂R antagonists in adult male Sprague–Dawley rats.

Parameters	AT2R antagonists
	EMA200	EMA300*	EMA400	EMA401*
i.v. administration
Dose (mg/kg i.v.)	7	10	1.0	1.0
C0 (ng/ml)	2837.4  ±  255.7	5736  ±  1468	12216  ±  1351	590.5
C0/dose (kg/ml)	405.3	573.6	2443.2	590.5
Kelim (h^−1)	0.22  ±  0.04	0.09  ±  0.01	0.12  ±  0.004	0.30
t1/2 (h)	3.6  ±  0.8	8.2  ±  1.0	5.9 ±  0.2	2.3
AUC0-t (ng.h/ml)	745.2  ±  31.2	1889.7  ±  655.1	1404.9  ±  94.1	868.2
AUC0-∞ (ng.h/ml)	758.3  ±  31.2	1942  ±  671	1424.9  ±  91.5	907
AUC0-∞/dose (h.kg/ml)	108.3	194.2	1424.9	907
Vd l/kg	47.3 ±  8.0	76.8  ±  30.3	6.0  ±  0.51	3.67
Cl l/h/kg	9.3  ±  0.4	6.1  ±  1.8	0.71  ±  0.05	1.10
Oral administration
Dose (mg/kg p.o.)	7	10	5	10
Cmax (ng/ml)	51.4  ±  1.2	67.7  ±  19.1	652 ±  182	885
Tmax (h)	0.23  ±  0.1	0.36  ±  0.13	0.33  ±  0.083	2.0
Cmax/dose (kg/ml)	7.3	6.8	130.4	88.5
Kelim (h^−1)	n.c.	0.067  ±  0.017	0.153  ±  0.006	0.122
t1/2 (h)	n.c.	11.9  ±  3.3	4.6	5.7
AUC0-t (ng.h/ml)	45.2  ±  6.5	123.9  ±  22.6	1951  ±  275.8	n.c.
AUC0-∞ (ng.h/ml)	n.c.	138.6  ±  25.3	1993  ±  269.1	3,010
AUC0-∞/dose (h.kg/ml)	6.5^‡	13.9	398.6	301
F (%)	5.9	7.1	28  ±  3.6	33.2

Adapted with permission from John Wiley and Sons [29].

*Sodium salt.

^‡Value based on AUC_0-t for EMA200.

AT₂R: Angiotensin II type 2 receptor; Cl: Systemic clearance; C_max: Peak plasma concentration after oral dosing; C₀: Extrapolated plasma concentration at time 0 after i.v. dosing; F (%): Oral bioavailability; h: Hour; i.v.: Intravenous; K_elim: Elimination rate constant; n.c.: Not calculable; p.o.: Oral; SEM: Standard error of the mean; t½: Elimination half-life; T_max: Time of peak plasma concentration after oral dosing; V_d: Volume of distribution.

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