Everyone knows how unpleasant itch can be, and the relief that comes from scratching. But when it comes to the causes of itch, while acute itch is commonly associated with skin irritation, researchers know less about what leads to chronic itch, a condition for which currently available treatments often don’t work. Now, a new study improves upon a previous strategy to treat itch, focusing on a well-known itch receptor.
Researchers led by first author Hans Jürgen Solinski, National Institute of Dental and Craniofacial Research, National Institutes of Health (NIH), Bethesda, US, in collaboration with James Inglese’s lab, National Center for Advancing Translational Sciences, NIH, set their sights on natriuretic peptide receptor 1 (NPR1), a receptor for the neuropeptide natriuretic polypeptide b (NPPB), that is expressed on itch-sensing neurons in the spinal cord. While a previous attempt to inhibit this receptor with an antagonist was unsuccessful in treating acute itch, Solinski and his team identify a new NPR1 antagonist that was able to inhibit itch in acute and chronic mouse models of the condition. Further, experiments with human dorsal root ganglion (DRG) neurons show that an NPR1-targeting approach has translational potential.
“This paper is important to different groups for different reasons,” said Steve Davidson, who studies itch as well as pain at the University of Cincinnati, US, but was not involved in the current work. “It’s important to drug developers because it provides an excellent validation of a molecular skeleton to potentially build new antipruritics that are going to work. It is important to basic pain scientists because it verifies previous basic science work showing that NPPB and NPR1 are working together in human neurons, and validates the antagonism of NPR1 as a target for antipruritics.”
The study was published July 10, 2019, in Science Translational Medicine.
(NPPB expression) of mice and men
Previous work from Santosh Mishra and current study senior author Mark Hoon identified NPR1 as a potential target for the treatment of itch (see related PRF news here). They saw that knockout of Nppb, as well as ablation of NPR1-expressing spinal cord interneurons, attenuated scratching responses to several itch-inducing agents in mice (Mishra and Hoon, 2013). The current study picks up on that earlier research, aiming to identify novel NPR1 inhibitors as potential treatments for acute and chronic itch.
But first, the researchers looked at whether human DRG neurons expressed NPPB. Quantitative polymerase chain reaction (qPCR) and in situ hybridization experiments revealed similar amounts of NPPB transcripts and NPPB-expressing neurons compared to what was previously reported in mice. The team also found that, as in mice, human NPPB-positive DRGs co-expressed the ion channel transient receptor potential vanilloid type 1 (TRPV1). These results suggested that itch neurotransmission in mice and people likely involved the same signaling molecules.
Explaining a past setback
Previous studies suggested that three different analogs of NPPB could inhibit NPR1 (Delporte et al., 1992; Kambayashi et al., 1989; Weber et al., 1991; Wyss et al., 1991). However, one of the analogs, called A-71915, was unable to attenuate acute itch in mice (Kiguchi et al., 2016). But in the current study, Solinski and his team found that A-71915 was not what it seemed, providing a possible explanation for its inability to relieve itch.
“It turned out that as well as being an antagonist, A-71915 was also a partial agonist in mice,” Hoon explained. “This gave us the feeling that perhaps a true antagonist of the receptor would block itch.”
So the researchers set out to search for better antagonists of NPR1. They used a quantitative high-throughput screening method to identify promising compounds from a large chemical library held at the National Center for Advancing Translational Sciences at the NIH.
Candidate compounds from the chemical library were prioritized by an automated robotic system based on their potency, efficacy, and structural relationships (Michael et al., 2008). This initial screen identified 1,408 compounds for further study. The researchers then used a series of overlapping strategies to remove any false positives and identify legitimate NPR1 inhibitors. This left 15 potential antagonists, and of these, three compounds—dubbed JS-5, JS-8, and JS-11—inhibited human NPR1 activity.
The cross-reactivity of those three compounds to a structurally similar family of enzymes was also examined, and based on those and other findings, the researchers concluded that JS-11 was a fairly selective antagonist of NPR1. A final in vitro assay confirmed inhibition of human NPR1 by the antagonists and revealed that they most likely blocked NPR1 through a noncompetitive mechanism. This means that the antagonists attached to a different part of the NPR1 receptor than where NPPB binds.
A new NPR1 antagonist inhibits itch in animals
Next, moving to in vivo work, the researchers tested whether NPR1 inhibition could alleviate itch in mice. Using a model of acute itch, they saw that intraperitoneal administration of JS-11 reduced scratching responses to an intradermal injection of histamine by more than half. This finding was corroborated using CYM5442, a second agent that elicits scratching in mice.
Importantly, intraperitoneal administration of JS-11 in mice did not lead to adverse effects on motor function, blood pressure, or heart rate, though further studies are required to examine potential side effects of chronic NPR1 antagonism. This is of great relevance, as NPR1 is expressed in a variety of other tissues, including the kidney (Potter et al., 2006).
The researchers also showed that JS-11 likely worked through a spinal cord mechanism. Here, the antagonist resulted in a decreased number of c-Fos-positive spinal cord neurons after the mice received intradermal histamine (c-Fos is a surrogate marker of neuronal activity). Intrathecal JS-11 also reduced scratching in mice response to histamine, in further support of a spinal cord locus of action.
Finally, to boost confidence that their strategy could work in people, the investigators showed in human DRG neurons that NPPB was co-expressed with other known itch receptors. They also saw that these neurons expressed the receptor for the cytokine interleukin-31 (IL-31). The IL-31 receptor plays a role in skin conditions associated with chronic itch, including atopic and contact dermatitis. The researchers also discovered that JS-11 decreased scratching in a mouse model of contact dermatitis.
Together, the findings suggest that NPR1 may serve as a potential target for treating acute and chronic itch. But with regard to JS-11 per se, the translatability of this agent is limited because of its low affinity for NPR1 and issues of cross-reactivity with other receptors. Preclinical testing in larger animals is also needed.
“There is a long and slow process involved in moving things forward,” Hoon explained. “Testing in non-human primates is the gold standard for drug development, and I know that there are European labs that have used pigs as models. But we have to go through quite a lot of steps before we get there.”
Lincoln Tracy, PhD, is a researcher and freelance writer based in Melbourne, Australia. Follow him on Twitter @lincolntracy.
This news piece was originally published on Pain Research Forum.