Clostridial toxin derivatives able to modify peripheral...

Details Clostridial toxin derivatives able to modify peripheral... Clostridial toxin derivatives able to modify peripheral...

1999-11-22

2002-05-28

Mosher, Mary E. (Department: 1648)

Chemistry: molecular biology and microbiology

Micro-organism, tissue cell culture or enzyme using process...

Recombinant dna technique included in method of making a...

C435S069100, C435S069700, C530S350000

Reexamination Certificate

active

06395513

ABSTRACT:

TECHNICAL FIELD
This invention relates to a novel agent that is able to modify peripheral afferent function. The agent may inhibit neurotransmitter release from discrete populations of neurons, and thereby reduce, or preferably prevent, the transmission of afferent pain signals from peripheral to central pain fibres. The agent may be used in or as a pharmaceutical for the treatment of pain, particularly chronic pain.
BACKGROUND
The sense of touch has traditionally been regarded as one of the five classical senses, but in reality it is highly complex, transducing a number of different sensations. These sensations are detected in the periphery by a variety of specialised nerve endings and associated structures. Some of these are specific for mechanical stimuli of various sorts such as touch, pressure, vibration, and the deformation of hairs or whiskers. Another class of nerves is able to detect temperatures, with different fibres being activated by heat and cold. A further population of nerve endings is not normally excited by mild stimuli, but by strong stimuli only. Sensory nerves of this category often respond to more than one stimulus, and are known as high-threshold polymodal fibres. They may be used to sense potentially damaging situations or objects. The polymodal fibres also transduce chemical signals such as the “burning” sensation evoked by acid. Thus, the sense of touch can transmit a very detailed description of objects and serve to both inform and warn of events.
The transduction of sensory signals from the periphery to sensation itself is achieved by a multi-neuronal pathway and the information processing centres of the brain. The first nerve cells of the pathway involved in the transmission of sensory stimuli are called primary sensory afferents. The cell bodies for the primary sensory afferents from the head and some of the internal organs reside in various of the ganglia associated with the cranial nerves, particularly the trigeminal nuclei and the nucleus of the solitary tract. The cell bodies for the primary sensory afferents for the remainder of the body lie in the dorsal root ganglia of the spinal column. The primary sensory afferents and their processes have been classified histologically; the cell bodies fall into two classes: A-type are large (60-120 &mgr;m in diameter) while B-type are smaller (14-30 &mgr;m) and more numerous. Similarly the processes fall into two categories: C-fibres lack the myelin sheath that A-fibres possess. A-fibres can be further sub-divided into A&bgr;-fibres, that are large diameter with well developed myelin, and A&dgr;-fibres, that are thinner with less well developed myelin. It is generally believed that A&bgr;-fibres arise from A-type cell bodies and that A&dgr;- and C-fibres arise from B-type cell bodies. These classifications can be further extended and subdivided by studying the selective expression of a range of molecular markers.
Functional analyses indicate that under normal circumstances A&bgr;-fibres transmit the senses of touch and moderate temperature discrimination, whereas the C-fibres are mainly equivalent to the polymodal high-threshold fibres mentioned above. The role of A&dgr;-fibres is less clear as they seem to have a variety of responsive modes, with both high and low thresholds.
After the activation of the primary sensory afferents the next step in the transduction of sensory signals is the activation of the projection neurons, which carry the signal to higher parts of the central nervous system such as the thalamic nuclei. The cell bodies of these neurons (other than those related to the cranial nerves) are located in the dorsal horn of the spinal cord. This is also where the synapses between the primary afferents and the projection neurons are located. The dorsal horn is organised into a series of laminae that are stacked, with lamina I being most dorsal followed by lamina II, etc. The different classes of primary afferents make synapses in different laminae. For cutaneous primary afferents, C-fibres make synapses in laminae I and II, A&dgr;-fibres in laminae I, II, and V, and A&bgr;-fibres in laminae III, IV, and V. Deeper laminae (V-VII, X) are thought to be involved in the sensory pathways arriving from deeper tissues such as muscles and the viscera.
The predominant neurotransmitter at the synapses between primary afferents and projection neurons is glutamate, although importantly the C-fibres contain several neuropeptides such as substance P and calcitonin-gene related peptide (CGRP). A-fibres may also express neuropeptides such as neuropeptide Y under some circumstances.
The efficiency of transmission of these synapses can be altered via descending pathways and by local interneurons in the spinal cord. These modulatory neurons release a number of mediators that are either inhibitory (e.g. opioid peptides, glycine) or excitatory (e.g. nitric oxide, cholecystokinin), to provide a mechanism for enhancing or reducing awareness of sensations.
A category of sensation that requires such physiological modulation is pain. Pain is a sensation that can warn of injury or illness, and as such is essential in everyday life. There are times, however, when there is a need to be able to ignore it, and physiologically this is a function of, for example, the opioid peptides. Unfortunately, despite these physiological mechanisms, pain can continue to be experienced during illnesses or after injuries long after its utility has passed. In these circumstances pain becomes a symptom of disease that would be better alleviated.
Clinically, pain can be divided into three categories: (1) Acute pain, usually arising from injury or surgery that is expected to disappear when that injury is healed. (2) Chronic pain arising from malignant disease; the majority of people with metastatic cancer have moderate to severe pain and this is resolved either by successful treatment of the disease or by the death of the patient. (3) Chronic pain not caused by malignant disease; this is a heterogeneous complaint, caused by a variety of illnesses, including arthritis and peripheral neuropathies, that are usually not life-threatening but which may last for decades with increasing levels of pain.
The physiology of pain that results from tissue damage is better understood than that which is caused by central nervous system defects. Under normal circumstances the sensations that lead to pain are first transduced by the A&dgr;- and C-fibres that carry high threshold signals. Thus the synapses in laminae I and II are involved in the transmission of the pain signals, using glutamate and the peptides released by C-fibres to produce activation of the appropriate projection neurons. There is, however, evidence that in some chronic pain states other A-fibres (including A&bgr;-fibres) can carry pain signals, and thus act as primary nociceptive afferents, for example in the hyperalgesia and allodynia associated with neuropathic pain. These changes have been associated with the expression of peptides such as neuropeptide Y in A fibres. During various chronic pain conditions the synapses of the various sensory afferents with projection neurons may be modified in several ways: there may be changes in morphology leading to an increase in the number of synapses, the levels and ratios of the different peptides may change, and the sensitivity of the projection neuron may change.
Given the enormity of the clinical problem presented by pain, considerable effort has been expended in finding methods for its alleviation. The most commonly used pharmaceuticals for the alleviation of pain fall into two categories: (1) Non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin and ibuprofen; (2) Opioids, including morphine.
NSAIDs have their main analgesic action at the periphery by inhibiting the production of prostaglandins by damaged tissues. Prostaglandins have been shown to be peripheral mediators of pain and inflammation and a reduction in their concentration provides relief to patients. This is especially the case in mild arthritic disease, where inflammat

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