Realizing the potential therapeutic value of deorphanization

This second phase is by far the most challenging, this involves using the chemical tools (agonist/antagonist) for the orphan receptor in disease or mechanistically relevant physiological assays to evaluate the role of the receptor and its potential as a therapeutic target for drug discovery. It is essential to have a detailed knowledge of the receptor's expression at the cell level, in both normal and diseased tissue to aid in directing functional studies most efficiently. We and others discovered a novel histamine receptor H4 (H4R) (Oda et al. 2000), which demonstrated a restricted expression pattern, predominately in peripheral blood leukocytes. A paper was identified describing a novel histamine receptor on eosinophils (Raible et al. 1995) whose pharmacology could not be rationalized in terms of H1, H2 or H3 receptor characteristics, that is, an example of'missing pharmacology^ Continuation of our studies with relevant in-vitro assays demonstrated a role for the histamine H4 receptor in histamine driven eosinophil chemotaxis, as such it is a potential target in inflammatory diseases (Fig. 10.3). Briefly, chemotaxis studies on isolated human eosinophils confirmed that histamine is chemotactic, and that selective agonists of the known histamine receptors (H1, H2, and H3) do not induce such a response. Furthermore, studies employing histamine-receptor antagonists showed only H3 antagonists clobenpropit and thioperamide inhibited chemotaxis due to their mixed H3 /H4 pharmacology. Since these compounds are also antagonists of the H4 receptor we postulated that the receptor mediating histaminergic chemotaxis is the histamine H4 (O'Reilly et al. 2002).

Alternative target validation tools include transgenic mouse knockout lines. These can be useful in evaluating receptor function, although care is needed when evaluating these results as species difference in receptor expression may be unrepresentative of man. Using the histamine H4 receptor as an example, it has been reported that the murine histamine H4 receptor demonstrates CNS expression specifically in the hippocampus (Zhu et al. 2001). However, in the reports concerning the human histamine H4, we and others (Oda etal. 2000;

Histamine H4: from EST to function

Determination of the IC50 for H3 antagonists of histamine-induced human eosinophil Chemotaxis

Eosinophil chemotaxis % of control

Existing literature is suggestive of 'missing' pharmacology

Eosinophil chemotaxis % of control

Existing literature is suggestive of 'missing' pharmacology

Clobenpropit dihydrobromide 1.5 |iM Thioperamide maleate 0.8|M

Histamine Dihydrochloride Conc.=30 nM

No agonist activity of Clobenpropit dihydrobromide or thioperamide maleate at 10 |M

Cells were pretreated with 5 ng/ml IL—5

Cells were pretreated with 5 ng/ml IL—5

Functional response in eosinophils il

Functional response in eosinophils il

Compounds

Receptor sub-type selectivity

Recombinant hH4R

(Pfi-013) pKi (mean±SEM)

Native hH3R pKi (mean±SEM)

Pyrilamine

(H1 antagonist)

<45

5.01 ±0.18

Cimetidine

(H2 antagonist)

5.03 ±0.11

<4

Thioperamide

(H3 antagonist)

7.26±0.06

6.79±0.10

Clobenpropit

(H3 antagonist)

8.18±0.02

8.63±0.04

lodophenpropit

(H3 antagonist)

7.87±0.04

7.94±0.10

Histamine

(Non-selective Agonist)

8.36±0.04

8.28±0.05

a-methylhistamine

(H3 agonist)

6.76±0.05

Histamine H4 may play role in eosinophil function

Hypothesis:

Histamine H4 may play role in eosinophil function

H3 & H4 share pharmacology Fig. 10.3 Describes the isolation and characterization of the human histamine H4 receptor. The pharmacological profile of H4 and the 'missing pharmacology' as described by Raible et al. (1994) led us to test the hypothesis and demonstrate that H4 may play a role in eosinophil chemotaxis.

O'Reilly et al. 2002) have not been able to demonstrate any expression of this receptor in the brain.

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