Peptides and compounds that bind to a thrombopoietin receptor

Details Peptides and compounds that bind to a thrombopoietin receptor Peptides and compounds that bind to a thrombopoietin receptor

2000-04-13

2002-10-15

Davenport, Avis M. (Department: 1653)

Drug, bio-affecting and body treating compositions

Designated organic active ingredient containing

Peptide containing doai

C514S014800, C514S015800, C424S185100

Reexamination Certificate

active

06465430

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention provides peptides and compounds that bind to and activate the thrombopoietin receptor (c-mpl or TPO-R) or otherwise act as a TPO agonist. The invention has application in the fields of biochemistry and medicinal chemistry and particularly provides TPO agonists for use in the treatment of human disease.
Megakaryocytes are bone marrow-derived cells, which are responsible for producing circulating blood platelets. Although comprising <0.25% of the bone marrow cells in most species, they have >10 times the volume of typical marrow cells. See Kuter et. al.
Proc. Natl. Acad. Sci. USA
91:11104-11108 (1994). Megakaryocytes undergo a process known as endomitosis whereby they replicate their nuclei but fail to undergo cell division and thereby give rise to polyploid cells. In response to a decreased platelet count, the endomitotic rate increases, higher ploidy megakaryocytes are formed, and the number of megakaryocytes may increase up to 3-fold. See Harker
J. Clin. Invest.
47:458-465 (1968). In contrast, in response to an elevated platelet count, the endomitotic rate decreases, lower ploidy megakaryocytes are formed, and the number of megakaryocytes may decrease by 50%.
The exact physiological feedback mechanism by which the mass of circulating platelets regulates the endomitotic rate and number of bone marrow megakaryocytes is not known. The circulating thrombopoietic factor involved in mediating this feedback loop is now thought to be thrombopoietin (TPO). More specifically, TPO has been shown to be the main humoral regulator in situations involving thrombocytopenia. See, e.g., Metcalf
Nature
369:519-520 (1994). TPO has been shown in several studies to increase platelet counts, increase platelet size, and increase isotope incorporation into platelets of recipient animals. Specifically, TPO is thought to affect megakaryocytopoiesis in several ways: (1) it produces increases in megakaryocyte size and number; (2) it produces an increase in DNA content, in the form of polyploidy, in megakaryocytes; (3) it increases megakaryocyte endomitosis; (4) it produces increased maturation of megakaryocytes; and (5) it produces an increase in the percentage of precursor cells, in the form of small acetylcholinesterase-positive cells, in the bone marrow.
Because platelets (thrombocytes) are necessary for blood clotting and when their numbers are very low a patient is at serious risk of death from catastrophic hemorrhage, TPO has Potential useful application in both the diagnosis and the treatment of various hematological disorders, for example, diseases primarily due to platelet defects. Ongoing clinical trials with TPO have indicated that TPO can be administered safely to patients. in addition, recent studies have provided a basis for the projection of efficacy of TPO therapy in the treatment of thrombocytopenia, and particularly thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transplantation as treatment for cancer or lymphoma. See, e.g., McDonald (1992)
Am. J. Ped. Hematology/Oncology
14:8-21 (1992).
The gene encoding TPO has been cloned and characterized. See Kuter et al.
Proc. Natl. Acad. Sci. USA
91:11104-11108 (1994); Barley et al.
Cell
77:1117-1124 (1994) ; Kaushansky et al.
Nature
369:568-571 (1994); Wendling et al.
Nature
369:571-574 (1994); and Sauvage et al.
Nature
369:533-538 (1994). Thrombopoietin is a glycoprotein with at least two forms, with apparent molecular masses of 25 kDa and 31 kDa, with a common N-terminal amino acid sequence. See, Bartley et al.
Cell
77:1117-1124 (1994). Thrombopoietin appears to have two distinct regions separated by a potential Arg-Arg cleavage site. The amino-terminal region is highly conserved in man and mouse, and has some homology with erythropoietin and interferon-a and interferon-b. The carboxy-terminal region shows wide species divergence.
The DNA sequences and encoded peptide sequences for human TPO-R (also known as c-mpl) have been described. See Vigon et al.
Proc. Natl. Acad. Sci. USA
89:5640-5644 (1992). TPO-R is a member of the haematopoietin growth factor receptor family, a family characterized by a common structural design of the extracellular domain, including four conserved C residues in the N-terminal portion and a WSXWS motif (SEQ ID NO:1) close to the transmembrane region. See Bazan
Proc. Natl. Acad. Sci. USA
87:6934-6938 (1990). Evidence that this receptor plays a functional role in hematopoiesis includes observations that its expression is restricted to spleen, bone marrow, or fetal liver in mice (see Souyri et al.
Cell
63:1137-1147 (1990)) and to megakaryocytes, platelets, and CD34
+
cells in humans (see Methia et al.
Blood
82:1395-1401 (1993)). Furthermore, exposure of CD34
+
cells to synthetic oligonucleotides antisense to mpl RNA significantly inhibits the appearance of megakaryocyte colonies without affecting erythroid or myeloic colony formation. Some workers postulate that the receptor functions as a homodimer, similar to the situation with the receptors for G-CSF and erythropoietin.
The availability of cloned genes for TPO-R facilitates the search for agonists of this important receptor. The availability of the recombinant receptor protein allows the study of receptor-ligand interaction in a variety of random and semi-random peptide diversity generation systems. These systems include the “peptides on plasmids” system described in U.S. Pat. Nos. 5,270,170 and 5,338,665; the “peptides on phase” system described in U.S. patent application Ser. No. 07/718,577, filed Jun. 20, 1991, U.S. patent application Ser. No. 07/541,108, filed Jun. 20, 1990, and in Cwirla et al.,
Proc. Natl. Acad. Sci. USA
87:6378-6382 (1990); the “polysome” system described in U.S. patent application Ser. No. 08/300,262, filed Sep. 2, 1994, which is a continuation-in-part application based on U.S. patent application Ser. No. 08/144,775, filed Oct. 29, 1993 and PCT WO 95/11992; the “encoded synthetic library” system described in U.S. patent application Ser. No. 08/146,886, filed Nov. 12, 1993, Ser. No. 07/946,239, filed Sep. 16, 1992, and Ser. No. 07/762,522, filed Sep. 18, 1991; and the “very large scale immobilized polymer synthesis” system described in U.S. Pat. No. 5,143,854, PCT Patent Publication No. 90/15070, published Dec. 13, 1990; U.S. patent application Ser. No. 07/624,120, filed Dec. 6, 1990; Fodor et al.
Science
251:767-773 (February/1991); Dower and Fodor
Ann. Rep. Med. Chem.
26:271-180 (1991); and U.S. patent application Ser. No. 07/805,727, filed Dec. 6, 1991; each of the foregoing patent applications and publications is incorporated herein by reference.
The slow recovery of platelet levels in patients suffering from thrombocytopenia is a serious problem, and has lent urgency to the-search for a blood growth factor agonist able to accelerate platelet regeneration. The present invention provides such an agonist.
SUMMARY OF THE INVENTION
This invention is directed, in part, to the novel and unexpected discovery that defined low molecular weight peptides and peptide mimetics have strong binding properties to the TPO-R and can activate the TPO-R. Accordingly, such peptides and peptide mimetics are useful for therapeutic purposes in treating conditions mediated by TPO (e.g., thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transfusions) as well as for diagnostic purposes in studying the mechanism of hematopoiesis and for the in vitro expansion of megakaroycytes and committed progenitor cells.
Peptides and peptide mimetics suitable for therapeutic and/or diagnostic purposes have an IC
50
of about 2 mM or less, as determined by the binding affinity assay set forth in Example 3 below wherein a lower IC
50
correlates to a stronger binding affinity to TPO-R. For pharmaceutical purposes, the peptides and peptidomimetics preferably have an IC
50
of no more than about 100 &mgr;m, more preferably, no more than 500 nM. In a preferred embodiment, the molecular weight of the peptide or peptide mime

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