Adenosine is a purine nucleoside that can signal through four distinct receptors (A1
R and A3
R; also known as ADORA1, ADORA2A, ADORA2B and ADORA3). Of these receptors, A1
R has received the greatest attention in pain-related studies. A1
R is a Gi/o
-coupled receptor that is expressed in nociceptive (pain-sensing) neurons, spinal cord neurons and other cells of the body [1
]. Agonists of this receptor have well-studied antinociceptive (see Glossary) effects in rodents when injected intrathecally, including antihyperalgesic and antiallodynic effects [5
]. Furthermore, intrathecal adenosine reduces allodynia and hyperalgesia in patients with chronic pain [6
]. In some instances, these pain-relieving effects in humans persist for days to months [7
Although adenosine has antinociceptive effects in humans [5
] and is being studied clinically (), adenosine and adenosine receptor agonists are not currently used to treat chronic pain in patients. There are several possible reasons for this. For one, there is uncertainty as to whether adenosine or adenosine receptor agonists treat spontaneous pain, a common symptom of chronic pain. In addition, adenosine has limited efficacy in patients (at the doses tested so far) and side effects (transient low back pain, headache) when injected intrathecally as a bolus [9
]. These side effects might result from the activation of adenosine A2
receptors—receptors that have pronociceptive (—pain-producing ) and vasodilatory effects when activated peripherally [1
]. Adenosine also reduces heart rate when administered intravenously, so there are serious cardiovascular obstacles that must be overcome if adenosine receptor agonists are to be administered systemically (Box 1
Recent clinical trials that target adenosine receptors associated with chronic pain (clinicaltrials.gov).
Box 1. Cardiovascular side effects of adenosine
One of the main challenges associated with developing adenosine receptor agonists as analgesics is the need to minimize cardiovascular side effects. A1
R is expressed in the atrioventricular and sinoatrial nodes of the heart, and A1
R activation in these regions can appreciably slow heart rate. The FDA approved drug Adenocard® (an intravenous formulation of adenosine) exploits this aspect of cardiovascular physiology to treat supraventricular tachycardia—an irregularly fast heartbeat. Based on this physiology, a number of full A1
R agonists were developed and tested as antiarrhythmics. Unfortunately, full A1
R agonists cause high-grade atrioventricular block and other serious cardiovascular side effects [85
]. In turn, these side effects have made it impractical to treat pain with systemic (oral or intravenous) full A1
In contrast to full agonists, partial A1
R agonists have modest to no effects on cardiovascular function when delivered systemically [85
]. Partial agonists activate receptors at submaximal levels and, unlike full agonists, typically do not desensitize receptors. Encouragingly, two partial A1
R agonists (CVT-3619-delivered orally; and INO-8875, formerly PJ-875-delivered intraocularly) were well tolerated and had no serious side effects in Phase I clinical trials for indications unrelated to pain. Different partial A1
R agonists given systemically had antihyperalgesic effects in a neuropathic pain model with reduced hemodynamic/cardiovascular side effects [86
These studies collectively suggest that cardiovascular side effects represent a real but surmountable challenge when targeting A1
R for analgesic drug development. Further testing of drugs that modulate A1
R in preclinical pain models is certainly warranted. Future efforts could include intrathecal injections as well as local peripheral injections of A1
R agonists, partial A1
R agonists or adenosine-generating ectonucleotidases. While some would argue that the market for intrathecal or local injection therapeutics is small relative to that which can be served with an orally active pill, such injections are more likely to avoid A1
R-associated cardiovascular side effects while also engage A1
R at spinal or axonal sites. In light of the numerous preclinical studies showing that A1
R activation is antinociceptive [5
], successful and selective targeting of A1
R could serve to benefit the patients who suffer daily from chronic pain.
Recently, several preclinical studies were published that further implicate adenosine receptors as targets for analgesic drug development. This review focuses on mechanistic insights that were gained from these studies and how insights from these studies could be leveraged to more discretely target adenosine receptors for pain control.
Effects on spontaneous pain
R activation reduces two common symptoms of chronic pain in humans and rodents, namely mechanical allodynia and thermal hyperalgesia [5
]. However, the most common and distressing symptom in patients is spontaneous pain [12
]. This includes symptoms like burning or stabbing pain. Unlike patients, rodents cannot directly communicate their feelings to human observers, rendering it a challenge to assess how experimental drugs affect spontaneous pain in lab animals.
Recently, King and colleagues developed a conditioned place preference assay to study spontaneous pain-like behaviors in rats [13
]. This assay is based on the assumption that relief of spontaneous pain in rodents is rewarding. As such, this assay can only be performed with drugs that are not intrinsically rewarding (thus excluding opioid analgesics which have reward and addiction potential). Using this model, King and colleagues found that clonidine and ω-conotoxin (ziconotide) reduced allodynia and spontaneous pain-like responses following nerve injury. In contrast, intrathecal adenosine reduced allodynia following nerve injury but did not inhibit spontaneous pain-like behaviors. These findings suggest that adenosine engages its targets centrally, because of its effects on allodynia, but does not engage neural pathways responsible for spontaneous pain-like responses. However, given that adenosine and ω-conotoxin act via a similar mechanism (both inhibit N-type calcium channels [14
]), these contrasting results raise the question–why was ω–conotoxin effective at inhibiting spontaneous pain-like responses while adenosine was not?
In a different study designed to evaluate how drugs affect spontaneous pain-like responses in rodents, Martin and colleagues found that intrathecal clonidine reduced self-administration of the opioid heroin in rats whereas intrathecal adenosine did not [17
]. As in the King study, these findings suggest that clonidine but not adenosine can block spontaneous pain following nerve injury (note that this conclusion is based on the assumption that opioids reduce the unpleasant/painful feelings associated with noxious stimuli in rodents). Likewise, in humans with neuropathic pain, intrathecal adenosine had no effect on spontaneous pain but reduced hyperalgesia and allodynia [18
], suggesting that adenosine does not inhibit spontaneous neuropathic pain; however, in a surgical pain model, spinal A1
R (but not A2A
R) activation did inhibit spontaneous (nonevoked) pain [19
]. It is thus possible that adenosine inhibits spontaneous pain in some conditions but not others. Such differences are not surprising given that the molecular and cellular mechanisms underlying various chronic pain conditions differ [20
]. An ongoing clinical trial comparing the analgesic efficacy of clonidine to adenosine may shed additional light on how these drugs affect spontaneous pain in humans (). Furthermore, new behavioral measures, like the mouse grimace scale [21
], could be employed to study how adenosine receptor agonists inhibit additional spontaneous pain-like behaviors.
Preemptive analgesic effects of adenosine
Several groups found that adenosine administered intravenously shortly before and during major surgeries provided long-lasting postoperative pain relief and reduced the need for postoperative opioid analgesics [6
], highlighting a preemptive analgesic effect of adenosine. However, in a recent double-blind clinical trial, intravenous adenosine had no preemptive analgesic effects in a similar surgical setting [23
]. Why this recent study failed when others succeeded is unclear, but could reflect differences in adenosine doses or differences in the timing of adenosine administration relative to incision.
In mice, prostatic acid phosphatase (PAP), an ectonucleotidase that generates adenosine over a three day period, provided an enduring (8 day) reduction in nociceptive sensitization if administered intrathecally one day before nerve injury or inflammation [24
]. These preemptive antinociceptive effects of PAP were attributed to sustained A1
R activation followed by depletion of phosphatidylinositol 4,5-bisphosphate (PIP2
), an essential signaling molecule that is required for pronociceptive receptors to sensitize neurons (). These mechanistic findings in rodents indicate that A1
R in nociceptive neurons and possibly in spinal neurons must be activated before sensitization is initiated. Extrapolating these animal studies to patients, preemptive analgesia might thus require A1
R activation before surgery (tissue damage) and maintained A1
R activation during surgery.
Figure 1 Proteins that regulate extracellular adenosine levels also influence adenosine receptor activation. (a). ATP can be released from neurons and/or glial cells by vesicular and non-vesicular mechanisms. Vesicular nucleotide transporter (VNUT/ SLC17A9), volume (more ...)
In the double-blind clinical trial showing no preemptive analgesic effects of intravenous adenosine, adenosine was administered at the time of incision until the end of surgery [23
]. Since adenosine infusion began coincident with surgery, PIP2
levels were likely at normal levels when surgery was initiated. As a result, the —inflammatorysoup of chemicals released following tissue damage [20
] would be expected to engage PIP2
-dependent signaling and transcriptional mechanisms at normal levels [25
]. Given that adenosine had preemptive analgesic effects in several clinical trials but not in this trial, additional clinical trials to resolve these differences are warranted. In any new trials, adenosine should be administered well before surgery initiation and one should make sure that adenosine receptors do not desensitize over the course of administration. It may also be worth comparing the preemptive analgesic effects of adenosine with other drugs that activate A1
R selectively or for longer durations, such as ectonucleotidases (intrathecally), A1
R agonists (provided they don’t cause receptor desensitization), or A1
R partial agonists (which do not as readily desensitize receptors). These latter drugs may have more reproducible and larger preemptive analgesic effects when compared to adenosine.