From 21-24 September, Hamburg, Germany, hosted the largest-yet Pain in Europe meeting, drawing 4,000 pain researchers and clinicians for the European Federation of IASP Chapters (EFIC) congress. Being a lone reporter, your PRF correspondent could take in only a fraction of the excellent talks and posters at the three-day meeting. This report on cancer and bone pain is the first in a series covering some of the action. If you were there and saw a compelling presentation or interesting poster, please consider sharing it—leave a comment below or send us your write-up!
Animal models lead the way
Cancer pain has a number of origins: It can be inflammatory, neuropathic, or stem from tumor-nerve interactions. It can even result from treatment, as with chemotherapy-induced neuropathies. Much cancer pain arises in bone, the preferred lodging spot for metastases of common cancers including breast and prostate. Several talks focused on bone pain in cancer and some promising new therapeutic targets.
The problem of bone pain from metastatic cancer is one where animal models have been particularly informative. Patrick Mantyh, University of Arizona, Tucson, originally developed an animal model for bone metastasis, now widely used. His lab has worked to elucidate the role of nerve growth factor (NGF) in cancer pain, including preclinical studies of anti-NGF antibodies.
As Mantyh explained, bone has a unique and restricted pattern of sensory innervation compared to skin. The periosteum tissue that surrounds bone contains the majority of bone-associated nerve fibers, with the bone marrow and mineralized bone receiving a significant, but much less dense, innervation and normal cartilage, none at all. The phenotype of nerve fibers in bone is also much different from skin: There are fewer types, with no rapidly conducting Aβ fibers or nonpeptidergic C fibers in bone. Most (80 percent) of the sensory afferents are peptidergic C fibers that express the NGF receptor TrkA, compared to 30 percent of fibers that are TrkA positive in skin (Castañeda-Corral et al., 2011). This may explain the reported potency of anti-NGF treatments in bone pain, Mantyh said. For example, in a Phase 2 study in osteoarthritis (OA), an anti-NGF antibody caused a profound 40-50 percent reduction in knee pain while walking (Lane et al., 2010).
In Mantyh’s model, fluorescently labeled tumor cells are injected into the femur of mouse, then the site is capped and researchers follow the growth of tumor cells and accompanying pain. In this setting, Mantyh has shown that an anti-NGF antibody works better than morphine to quell pain, as evidenced by guarding and flinching behavior in the mice.
NGF is known to directly sensitize nerve fibers, accounting for its pain-promoting activity, but Mantyh and coworkers recently identified an additional action of NGF: The factor promotes sensory and sympathetic nerve fiber sprouting in tumor-bearing bones. In their mouse model, they see intense sensory and sympathetic nerve sprouting in the periosteum, mineralized bone and bone marrow, and even see some neuroma-like structures. The overgrowth is blocked by the anti-NGF antibody (Mantyh et al., 2010). Mantyh hypothesized that this sprouting might explain why pain can become more severe with cancer progression.
Does sprouting and the development of mixed-fiber bundles explain breakthrough pain, the extreme spontaneous pain that can occur in cancer patients even when they are taking pain medication regularly? Breakthrough pain resembles that seen with neuromas, Mantyh said. His group is now looking at whether the targeting of sodium channels can reduce the sprouting-associated pain as it does for neuromas.
Recently, the lab showed that anti-NGF is able to attenuate sprouting and reduce pain even when it is given late after tumor inoculation, when the pathological sprouting has already occurred (Jimenez-Andrade et al., 2011). This indicates that tumor-induced sensory remodeling is a dynamic process, according to Mantyh. As parts of tumors become necrotic, the associated nerve fibers die, but new fibers sprout at the growth edge. Interrupting this new growth, even in an established tumor, leads to an improvement in pain, he reasons.
Where is the NGF coming from? Original studies used sarcoma cells, which make copious amounts of NGF. But Mantyh and colleagues have since found that a prostate tumor cell that does not make NGF can also induce ectopic sprouting. In that case, the source of the NGF appears to be stromal cells (Jimenez-Andrade et al., 2010). A similar situation occurs in breast cancer, they showed (Bloom et al., 2011).
NGF also plays a role in nonmalignant pain, such as in aging joints, and in fracture. The skeleton changes over life, and, Mantyh said, “As skeleton remodels, nerves remodel, too, and can remodel in a pathological way.” He has shown that NGF plays a role in fracture pain, and that a TrkA inhibitor relieves fracture pain better than morphine in a mouse femur fracture model (Ghilardi et al., 2011).
In the end, Mantyh’s message was that NGF does play a major role in skeletal pain, and anti-NGF treatments work well to quell pain. But due to adverse events in Phase 3 trials of anti-NGF antibodies—in the OA study, 16 patients needed joint replacement when their arthritis worsened—all of the trials except the cancer studies are on hold while researchers and regulators try to figure out if or how blocking NGF leads to joint failure.
Uncovering other factors
NGF may be important, but it is not the whole story in cancer pain. In a talk and poster, Rohini Kuner, University of Heidelberg, Germany, presented new data on the role for the vascular endothelial cell growth factor (VEGF) family of ligands in cancer pain. Her data showed that, like NGF, VEGF receptor (VEGFR) ligands can act directly on nerves to sensitize them, and also to induce sprouting. The results suggest that VEGF ligands are another mediator of tumor-nerve interactions, and that they contribute to pain. The results could have immediate clinical application, she said, because the peptides that block VEGF signaling are available, and could be administered locally. The new work continues a theme from Kuner’s lab, which recently reported another novel mediator of tumor-nerve-immune interactions, namely, the hematopoietic factors G-CSF and GM-CSF that are made by tumors and interact with receptors on peripheral nerves to cause pain (Schweizerhof et al., 2009; Stösser et al., 2011).
Moving away from growth factors, Stephen McMahon, King’s College London, UK,made the case for ATP as a pain mediator in bone cancer pain. He showed that an antagonist of the purinoceptor P2X3 has analgesic activity in a rat model of bone pain, although the ultimate mechanism is unclear.
Receptors for extracellular ATP fall into two classes: The P2X group (seven receptors) are ionotrophic, while the P2Y group (eight receptors) are metabotropic G protein-coupled receptors. The X family receptors are trimeric, and can exist in many different combinations. Of particular interest are the X3-containing receptors, which are expressed in 35 percent of DRG neurons and appear to be restricted to a subset of primary nociceptors. The X3 receptors are highly expressed in non-peptidergic C fibers, fewer in CGRP-positive neurons, and absent in Aβ fibers. The receptors mediate nociceptive pain signals: In rat, 50 percent of nociceptors respond to the P2X3 agonist α,βmeATP with neuronal firing. In human pain models, ATP ionophoresed into skin produces a mild burning sensation, and can potentiate pain in other models. The receptor “might be an interesting target if it were engaged in some real pain state,” McMahon said.
Last year, he and his colleagues showed that it is, demonstrating that the specific P2X3 antagonist AF-353 eased pain behaviors in a rat model of metastatic breast cancer (Kaan et al., 2010). The locus of action of the inhibitor seemed to be central terminals of afferent neurons in the dorsal horn, where it quieted neuronal activation. Data suggested that the inhibitor acted pre-synaptically to affect neurotransmitter release. At the same time, there may also be a peripheral action of ATP as well: The investigators showed that in mixed cultures, tumor cells released ATP that could stimulate ERK kinase signaling in co-cultured DRG neurons.
One idea is that ATP made peripherally drives afferent terminals, and P2X3 activation in the spinal cord gates neurotransmitter release. If the central actions turn out to be key, then it will be necessary to use brain-penetrant inhibitors, McMahon said.
Trouble from treatment
Not all cancer pain is bone pain—patients often develop neuropathy as a result of chemotherapy. In patients with metastatic cancer pain, a fifth of pain is treatment related, said Patrick Dougherty, University of Texas MD Anderson Cancer Center, Houston. Many chemotherapeutic agents cause neuropathy, most commonly with a stocking and glove distribution on glabrous (hairless) skin. The painful neuropathy affects all types of sensory fibers and does not reverse when treatment ends, but can persist for years. Chemotherapy is associated with loss of intraepidermal nerve fibers (ENFs), the bare nerve endings of sensory fibers in skin that are responsible for transmission of peripheral pain. In patients treated with chemotherapy, innervation goes to nothing.
Just having cancer already causes a subclinical neuropathy, Dougherty said. Patients have cold fingertips and show impairment in touch detection. When he took punch biopsies of fingertip skin and looked at ENFs before treatment, he saw a huge variation in baseline innervation. That led to the idea that chemotherapy represents a global neurotoxic injury, and people who start with a low baseline innervation will be at highest risk for neuropathy. Dougherty said he is now gearing up to test this idea in patients, using a non-invasive microscopy to visualize nerve fibers without the need for biopsy.
In animal models, treatment with a low dose of the chemotherapeutic agents oxaliplatin or taxol also causes a loss of ENFs, which correlates with the appearance of mechanical hyperalgesia. Both the fiber loss and hyperalgesia are prevented with treatment with the immunomodulator minocycline, Dougherty recently showed (Boyette-Davis and Dougherty, 2011; Boyette-Davis et al., 2011).
Why is the neuropathy painful? In animals with chemotherapy-induced neuropathy, electrophysiological recordings show that once neurons are induced to fire by a painful stimulus, the cells keep firing even when the stimulus is removed. That indicates a failure to stop excitatory transmission, but what is the mechanism?
One possibility is a tumor necrosis factor (TNF)-mediated loss of inhibitory neuronal activity. Dougherty had previously shown that TNF caused a loss of activity of inhibitory GABAergic interneurons in the dorsal horn of the spinal cord (Zhang et al., 2010), a loss that is thought to be important in the development and maintenance of neuropathic pain. More recently, the group found that this happens via TNF inhibition of the hyperpolarization-activated cation currents (Ih) (Zhang and Dougherty, 2011) that Dougherty said they think are mediated by the pacemaker channel HCN2 (see PRF related news story). The effect requires the type 1 TNF receptor (TNFR1), which is expressed in both neurons and astrocytes in the spinal cord, and proceeds via p38 map kinase activation. Dougherty proposed this as a novel central mechanism by which TNF affects pain signaling beyond its documented nociceptor sensitization effects.
In the taxol-treated animals, Dougherty reported increased expression of TNFR1 on astrocytes and neurons in the dorsal horn. The drug had other effects, too: a rapid and sustained activation of spinal astrocytes that was blocked by minocycline. The glutamate transporter GLAST was downregulated in astrocytes, an effect that might increase excitatory activity. The downregulation was also reversed by monocline. Microglia are activated, too, but only weakly and transiently.
The activation of astrocytes, but not microglia, differentiates the response to chemotherapy from what occurs in the spinal nerve ligation model of neuropathic pain, but Dougherty says that does not mean the astrocytes are causing the sensory changes. He can’t say whether the onset of sensory changes occurs as early as the astrocyte activation, for one. And he thinks the primary site of action of taxol is the DRG. “Chemotherapy doesn’t get into the CNS,” he said.
Were you at EFIC VII? Did you see additional interesting talks or posters relating to cancer or bone pain? If so, please let us know by leaving a comment.
