From August 27-31, pain researchers and clinicians from around the world gathered in Milan, Italy, for the 14th World Congress on Pain. With more than 7,700 attendees, the Congress was the largest yet. The days were crammed with plenary lectures, workshops, and hundreds of posters. This is the first of a series of reports from the meeting.
From high hopes, to uncertain prospects, to cautious optimism, the development of a new class of painkillers based on blocking nerve growth factor (NGF) has been a long and winding road. After decades of basic and preclinical research, stellar clinical trial results began appearing in 2005 for anti-NGF antibodies. Then, the antibodies’ fate was cast into limbo when unexpected safety concerns caused the US Food and Drug Administration (FDA) in 2010 to halt multiple late-stage clinical trials. Since then, much of the data from trials involving antibodies from three different companies have been released and reviewed, with the result that an FDA advisory committee recommended in March of 2012 that clinical testing be allowed to continue (see PRF related news story). In a session at the 14th World Congress on Pain, speakers reviewed the biology of the NGF pathway and the antibody data to date, and showed new findings on a novel small-molecule inhibitor of NGF signaling.
Stephen McMahon, King's College London, introduced NGF as the poster child for the promise, and challenges, of bench-to-bedside research. Work over the years from McMahon and others showed that NGF is an important peripheral pain mediator that is upregulated under pathological conditions, that activates and sensitizes peripheral nociceptors, and that also affects central sensitization. In 1995, McMahon and colleagues were among the first to speculate in a paper that NGF antagonists may be of clinical use (McMahon et al., 1995). Since then, a parade of studies in a variety of animal models from many labs has demonstrated that blocking NGF reduces pain. Driving the optimism about NGF-targeted therapies was the “sheer volume of activity from different labs without axes to grind,” McMahon said, that all converged on the idea than NGF plays an important role in different pain states.
The result was a race to take NGF-targeted therapies into the clinic, in which pharmaceutical giant Pfizer took the lead with its anti-NGF antibody, tanezumab. At the IASP meeting, Mark Brown, Pfizer Inc., New London, Connecticut, US, reviewed the results of clinical trials with tanezumab, which acts by sequestering NGF and preventing its binding to either of its two receptors, the tropomyosin-related kinase A (TrkA) and p75. Pfizer has conducted extensive Phase 3 testing of the antibody in osteoarthritis (OA), and Phase 2 testing for other pain conditions, most prominently low back pain. The efficacy of the antibodies in OA was striking (see Phase 2 results in Lane et al., 2010), but the FDA halted the massive clinical program in 2010 after some patients in the trials experienced rapidly worsening arthritis that necessitated joint replacement. Clinical trials have yet to resume, six months after an FDA advisory committee recommended in March that the agency lift the hold and allow continued development of the agents.
Brown presented summary data of the tanezumab clinical trials, all of which are publicly available on the FDA website. In 2010, Pfizer had 30 studies in progress, involving 11,079 patients, including 1,649 in placebo arms, 6,410 in monotherapy for up to two years, and 3,400 in combination therapy with non-steroidal anti-inflammatory drugs (NSAIDs), at doses of antibody ranging from 2.5 mg to 20 mg administered intravenously or subcutaneously every eight weeks. When the trials were put on hold, all dosing stopped and the patients were followed for safety.
Efficacy was clear: For OA, the antibody, alone or in combination with NSAIDs, gave statistically significant and clinically relevant reductions in pain and function in all trials (see also Brown et al., 2012). Either monotherapy or combination therapy was better than the cyclooxygenase-2 (COX2) inhibitor celecoxib alone, but the combination of antibody plus NSAID did not provide a clear advantage over monotherapy. In a trial comparing the antibody to oxycodone, the opioid performed no better than placebo, and tanezumab beat both.
Brown said that the 2.5 and 5 mg doses are emerging as the therapeutic dose in OA, while in chronic low back pain, higher doses (10 or 20 mg) were needed to see improvement over placebo or naproxen. Brown reported that, before the trials were stopped, there was also preliminary evidence of analgesic efficacy at 20 mg for neuropathic and visceral pain, but he did not show the data.
The clinical halt occurred after reports of unexpected painful joint destruction leading to total joint replacement in trial subjects. A look at Pfizer’s overall data showed that the incidence of joint replacements in the OA trials was not increased by antibody treatment alone compared to placebo, and ranged from 32 to 47 replacements per 1,000 person years, a level to be expected in this patient population. However, when antibody was combined with NSAIDs, there was a significant dose-dependent increase to 89-117 replacements per 1,000 person years.
Pfizer convened a panel of outside experts to examine the cases of all 386 trial subjects who required joint replacements. That panel concluded that in half, the cause was normal progression of OA, and in one-quarter it was rapidly progressing osteoarthritis (RPOA), a condition where joint space narrowing is followed by bone loss. Cases of RPOA were more frequent in tanezumab treatment groups compared to no-antibody groups, and were highest in the antibody plus NSAID group. Increasing doses of antibody or times of NSAID use were associated with higher rates of RPOA.
The reason for the increase in RPOA is not clear, but one idea is that, because pain relief is so effective, patients injure joints by overusing them. The data did not support that, however: Brown showed there was no correlation between the extent of pain relief and rapid progression among the different treatment groups. For example, antibody-only and antibody plus NSAID gave the same pain relief, while the antibody plus NSAID group had many more cases of RPOA. Even so, the findings do not exclude that substantial pain relief could lead to increased loading and accelerated damage in some arthritic joints where there is a preexisting weakness of the underlying bone. The situation could be worsened by NSAIDs, which may interfere with bone repair.
In agreement with the idea that arthritis sets up joints for damage, Brown said that only one case of RPOA occurred outside of an osteoarthritis trial, and that was in a low back pain subject who also had OA. “We have not seen RPOA in any other trials,” Brown said. “Those are much smaller populations, but the data are reassuring.”
The joint safety issues were not identified by any of the extensive preclinical research on NGF, Brown pointed out. Animal studies in multiple species showed no evidence of joint destruction, influence on cartilage, or progression of OA, even with weekly doses 100 times higher than used in the clinic. Weekly dosing for six months in monkeys did not reveal problems.
The researchers are now looking for animal models they might use to investigate the basis for the effect, and Brown referred to data presented at the FDA meeting by Regeneron Pharmaceuticals, Tarrytown, New York, US, that may offer some clues. According to that report (which is also available on the FDA website), biweekly administration of Regeneron’s anti-NGF (REGN475) antibody alone in a mouse model of arthritis did not result in statistically significant anatomical differences in bone or vascular markers, but REGN475 in combination with the NSAID indomethacin led to a significant reduction in cartilage area and density, and elevation of a serum marker of activity of bone-resorbing osteoclasts. “These data suggest a potential detrimental effect of co-administration of REGN475 with an NSAID on joint health,” the Regeneron researchers wrote. “As such, this animal model provides a potential means to explore the pathophysiological basis for the joint-related clinical findings that have been described with certain anti-NGF antibodies.” Brown said Pfizer is now working with the same model.
And, Brown said Pfizer believes they have identified measures to reduce the risk of RPOA in patients treated with tanezumab by 90 percent by avoiding NSAID use, keeping antibody doses below 10 mg, and excluding patients with RPOA or those who do not respond to a first dose of antibody.
Brown reported that Pfizer is currently negotiating with the FDA on the conditions for restarting their clinical tests; he believes the antibody will offer benefit “if and when we’re allowed to resume clinical development.”
Beyond antibodies
Before NGF antibodies, researchers had tried to target NGF signaling pathways with small molecule inhibitors of the NGF receptor TrkA. Compared to antibodies, such inhibitors offer some advantages: specific blockade of one receptor that has been shown to be important for pain rather than wholesale inhibition of NGF actions at both of its receptors, and an ability to titrate or rapidly reverse inhibition for dose adjustment or acute treatment. However, designing inhibitors that block TrkA specifically is a challenge: TrkA is one of a family of tyrosine kinase receptors that also includes TrkB (a receptor for brain-derived neurotrophic factor [BDNF] and neurotrophin 4/5) and TrkC (the receptor for neurotrophin 3). The preferred target for small molecules—the ATP binding pocket of the receptor kinase—is identical in all three family members, making it impossible to design selective inhibitors.
Steven Andrews, of Array BioPharma, Boulder, Colorado, US, outlined his company’s progress on TrkA inhibitors. Array and academic collaborators have previously published results using the pan-Trk inhibitor ARRY-470, which binds in the ATP pocket and is equipotent against the three Trks. The compound relieves pain in animal models of bone cancer (Ghilardi et al., 2010) and bone fracture (Ghilardi et al., 2011). Andrews showed additional preclinical data on a complete Freund’s adjuvant (CFA)-induced arthritis model, where ARRY-470 given orally twice daily provided better pain relief than did three NSAIDs, including naproxen.
However, the researchers did see several troublesome safety signs. The good news was that dosing of rodents for six weeks at 100 times the therapeutic dose produced no change in neuronal density in the brain or spinal cord, and no effects on normal pain sensation. But the compounds caused overeating (hyperphagia) and a dose-dependent dramatic increase in weight in the animals. Andrews said that, based on previous results, this is likely to be an effect of blocking central BDNF/TrkB signaling (Unger et al., 2007). They did not see the effect in monkeys, so it may be specific to rodents. They also observed ataxia (the mice looked drunk), which was mild at therapeutic doses but increased with higher doses. The effect tracked with the level of Trk inhibition in the CNS, and raised concerns about a narrow therapeutic index for the compounds, Andrews said. The results together suggest that the lack of specificity for Trk receptors does not affect pain relief, but can add unwanted side effects.
But how to make a TrkA-selective compound? Andrews presented new data with an allosteric inhibitor of the TrkA kinase that binds to a unique site outside of the ATP binding pocket and offers a chance to distinguish TrkA from its close relatives. Using a structure-based approach to design potent inhibitors (to date they have solved 120 receptor-ligand structures), they now have allosteric inhibitors with nanomolar potency, 1,000-fold selectivity for TrkA over TrkB and C, and 10,000-fold selectivity over other kinases.
In a CFA animal model of inflammatory pain, one of the compounds (AR-786) was equally as effective as the pan-Trk inhibitor, showing a quick onset and efficacy for eight hours. In a model of surgical incision pain, the compound gave long-lasting pain relief, and in the mono-iodoacetate arthritis model, pain relief was equivalent to the NSAID celecoxib. A second compound, AR-256, produced no hyperphagia, weight gain, or ataxia, even at doses 100 times the therapeutic dose. Andrews concluded that blocking TrkA at the allosteric site is sufficient to shut down the NGF signaling cascade, and that intermittent dosing of the inhibitor (as opposed to the long-lasting blockade with antibody) is sufficient for efficacy.
The activity of the inhibitors compared well with the NGF antibody in animal tests. “Preclinically, it seems we can achieve the efficacy of the antibody,” Andrews said. Array has multiple compounds with oral bioavailability, has identified a lead compound, and is now doing toxicology studies in preparation for human testing. They have not yet evaluated the inhibitors extensively in neuropathic pain, and are looking for academic collaborators to evaluate their tool compounds in additional animal models.
Outstanding questions
Much has been learned about NGF on its way to the clinic, but McMahon reminded the audience of a few crucial unknowns, beyond the looming question of why NGF antibodies appear to accelerate joint damage in people with arthritis.
One question is whether the second NGF receptor, p75, plays a role in pain or other effects of NGF. That could bear on whether TrkA-targeted or NGF-targeted therapies will work best. In addition, it has been suggested that the precursor of NGF, proNGF, may act preferentially via p75, but whether the ligand-receptor pair plays a role in pain is unknown.
When NGF binds to TrkA, it activates multiple intracellular signaling cascades that are not completely understood. Like other pain mediators, NGF triggers the post-translational modification of channels including TRPV1 and others that alter neuronal sensitivity to painful stimuli. NGF has unique functions as a neurotransmitter and neuromodulator of central pain as well.
A special feature of NGF is that, once it binds TrkA on nerve terminals, it is internalized and transported retrogradely to the cell body, where it causes changes in expression of genes for neurotrophins, receptors, and ion channels. The dramatic clinical effects of targeting NGF among the myriad pain mediators in chronic conditions suggest that NGF has a special role in pain, and McMahon said he suspects that the factor’s transcriptional effects are part of that.
That said, the full effect of transcriptional changes induced by NGF is unknown, and McMahon pointed out that no one has published a genomewide accounting of NGF-induced changes in gene transcription. His group started looking at that eight years ago but never published the data, which he said were incomplete. Recently, he took a new look at the data, seeking the most highly upregulated genes in dorsal root ganglia (DRG) neurons exposed to NGF. He saw the expected genes, but also some unexpected things. The pain mediator calcitonin gene-related peptide (CGRP) is elevated 1.6-fold, as expected, but approximately a thousand other genes are upregulated to at least that level as well. The nature and functional significance of most of those genes are unknown.
The biggest open question, McMahon believes, is the relative importance of transcriptional versus post-translational effects of NGF in generating and sustaining abnormal pain states. These and other questions are likely to be the subject of much interest as more NGF-targeted therapies make their way to clinical testing in the next few years.
NGF Update: On October 4, Mark Brown of Pfizer announced that the FDA will allow the company’s anti-NGF antibody, tanezumab, to re-enter clinical trials. “I can tell you that in our case we are off clinical hold, as of a couple weeks ago,” he said during a panel discussion at the Pain Therapeutics Summit in San Jose, California, US. David Upmalis of Janssen, also on the panel, could not say the same about his company’s antibody, fulranumab, but he was optimistic; lifting the hold on fulranumab is “imminent, if Pfizer’s there,” he said.--Megan Talkington
