Current Medicinal Chemistry - Immunology, Endocrine & Metabolic Agents - Volume 2, Issue 4, 2002

Advances in angiogenesis research have shed a new light on the growth and metastasic spread of solid tumors, allowing to define new paradigms for cancer treatment. These studies have highlighted the dramatic therapeutic potential of natural inhibitors of angiogenesis, which were found capable of maintaining tumors in a state of dormancy. One of the most promising of these recently described natural inhibitors of angiogenesis is endostatin, a C-terminal fragment of collagen XVIII. In-vitro, endostatin strongly inhibits endothelial cell proliferation and migration. Initial in-vivo studies were impressive, recombinant endostatin was shown to induce regression and prevent the growth of experimental tumors in mice. Several studies by independent teams were published thereafter, they either described different forms of the recombinant protein, or developed gene therapy approaches. Most groups have shown perceptible activity in mouse tumor models, albeit without evidence of tumor regression. More recent studies have failed to show any significant antitumor activity. The resolution of these paradoxes is fundamental for obtaining a better view of the therapeutic potential of endostatin. This may require a better understanding of the mechanism of action of endostatin at the molecular level, which remains largely unknown.

The common syndrome of insulin resistance with hyperinsulinemia is frequently associated with a collection of metabolic abnormalities, including dyslipidemia, hypertension and diabetes. Lipodystrophy syndromes, which include both genetic and acquired forms, resemble the insulin resistance syndrome, particularly with respect to the presence of associated biochemical disturbances such as hyperlipidemia and diabetes. Since the discovery in 1999 that mutant LMNA causes Dunnigan-type partial lipodystrophy, human mutations in three other genes have been implicated in inherited lipodystrophy syndromes, namely PPARG in partial lipodystrophy, and AGPAT and BSCL2 in complete lipodystrophy. There is evidence for additional genetic heterogeneity for lipodystrophy syndromes. Delineation of the human molecular genetic basis of forms of inherited lipodystrophy may help to identify pathways and molecular targets for the common insulin resistance syndrome and for acquired drug-induced lipodystrophy syndromes.

Primary adrenal insufficiency (PAI) is the consequence of the bilateral destruction or impaired function of the adrenal cortex. The reduction of adrenocortical cell mass is responsible for a deficiency of glucocorticoids and, in some cases, of mineralocorticoids and adrenal androgens. Over the past few decades, new data on the prevalence of PAI, advances in the procedure for the clinical testing of the adrenal function, and the development of novel genetic and molecular biology techniques have improved our understanding of the physiopathological mechanisms involved in the process of adrenal dysfunction and its genetic background. The estimated prevalence of PAI is 110-120 subjects per million individuals, being slightly higher among females than among males. The diagnosis of PAI is made in the presence of the typical symptoms and signs accompanied by reduced levels of serum cortisol and elevated levels of plasmatic ACTH. The intravenous injection of synthetic ACTH (250 μg) (HDT) is helpful for testing cortisol response in patients with suspected PAI. Recently, we have demonstrated that the intravenous injection of very low doses of synthetic ACTH (1 μg) (LDT) has a high diagnostic sensitivity and specificity for PAI, and this test could be proposed as an accurate way to identify subjects with adrenal dysfunction. The presence of a mineralocorticoid deficiency can be tested by measuring plasmatic or urinary aldosterone levels, which can be low in PAI, and plasma renin activity, which is elevated. Adrenal androgen deficiency (i.e. DHEA) can be assessed by specific radioimmunoassays, but they are not usually performed in routine clinical practice. The clinical spectrum of Addison's disease has expanded dramatically, and a long series of additional causative mechanisms has been identified. In Western countries, up to 70% of cases of PAI is the consequence of the autoimmune destruction of adrenocortical cells. The enzyme steroid 21-hydroxylase (21OH) is the major adrenal autoantigen in autoimmune PAI and the appearance of adrenal cell autoantibodies (ACA) and 21OH antibodies (21OHAb) is considered a sensitive and specific immune marker of this condition. The genetic risk for autoimmune PAI is associated with HLA-DR3-DQ2 and with allele 5.1 of the MHC class I chain-related A (MICA) gene. Chronic PAI may also result from targeting of the adrenal gland by Mycobacterium tuberculosis, fungal agents, Cytomegalovirus infection during AIDS or coagulative diseases and bilateral metastasis. Many genetic disorders, such as X-linked adrenoleukodystrophy (ALD), adrenal hypoplasia congenita (AHC) and ACTH resistance syndrome, can cause PAI in children, and sometimes also in adult subjects. ALD is a hereditary peroxisomal disorder characterized by demyelination of the nervous system and PAI, the detection of increased plasmatic levels of VLCFA is the biochemical marker of this disease. AHC is another X-linked disorder, in which affected males present with hypogonadotropic hypogonadism and PAI, the gene involved is known as DAX-1 and is located in the long arm of chromosome X. Familial ACTH resistance syndrome is characterized by high levels of ACTH and adrenal insufficiency due to mutations of the ACTH receptor gene, located on chromosome 18p11. This clinical form should be differentiated from triple A syndrome (achalasia, alacrimia, adrenal insufficiency), which is caused by a recently identified gene located on chromosome 12q13. This wide range of forms of PAI suggests that identification of the etiologic cause of PAI in the individual patient is an important diagnostic step, due to the different clinical management. The treatment of PAI is based on hormone-replacement therapy. Cortisol deficiency can be compensated by oral administration of hydrocortisone or cortisone acetate twice or three times a day. The mineralocorticoid drug used for aldosterone replacement is 9-alpha-fluoro-cortisol, administered orally every day or on alternate days. Recent clinical studies have shown that the administration of DHEA is beneficial in PAI patients, and for this reason it has been proposed that treatment with DHEA could be a useful supplement to the replacement therapy for PAI.

Although tissue-type plasminogen activator (t-PA) is believed to be beneficial in the treatment of acute strokes, t-PA treatment increases the risk of symptomatic cerebral hemorrhage. Therefore, the possibility of a fatal complication of cerebral hemorrhage may limit the potential benefits of anti-thrombotic and thrombolytic agents. The potential mechanisms of hemorrhage may be activated by an increase of plasmin activity that produces systemic hemostatic defects. Furthermore, plasmin activates matrix metalloproteinases (MMPs), which degrade extracellular matrix components including laminin and fibronectin. The impairment of vascular integrity and the production of hemorrhage then follow. It has been reported that MMP activity increased in hemorrhagic transformation after cerebral ischemia / reperfusion injury in primates, and that the activated forms of MMPs may play an essential role in opening the blood-brain barrier and in cerebral hemorrhage. Recently, two spin trap agents and a metalloproteinase inhibitor have been reported to reduce cerebral hemorrhage following thrombotic stroke. Although it is unclear by what mechanism(s) free radicals may increase MMP expression, these findings suggest that free radicals and MMPs may play essential roles in cerebral hemorrhage from the treatments with thrombolytic and anti-thrombotic agents in acute strokes. We demonstrated that a free radical scavenger inhibited cerebral hemorrhage produced by heparin, and that the combination of heparin and such a free radical scavenger reduced infarct size and improved neurological symptoms. Our observations point to combination therapy with thrombolytic or anti-thrombotic agents and a free radical scavenger having implications for treatment of acute stroke in human.

In the pursuit of new drug targets and novel drugs, finding a promising remedy for an incurable disease is an unusual feat. This exactly happened recently with the proteasome and its inhibitor, PS-341, which emerged in initial clinical trials as a prospective drug against multiple myeloma. The proteasome is the major executor of a tightly regulated nonlysosomal proteolysis in human cells and constitutes an attractive target for the development of drugs against cancer, autoimmune diseases, muscle wasting, inflammation and stroke. Synthetic peptide derivatives: boronates, epoxides, aldehydes, vinyl sulfones, cyclic peptides and lactones are tested for their in vivo and in vitro performance. These compounds block the N-terminal threonine-type active centers of the enzyme, halting cleavage of all proteasomal substrates in the cell and triggering apoptosis. Apparently, cancer cells are more susceptible than normal cells to such drastic treatment. The great advantage of competitive proteasome inhibitors as drugs derives from the apparent lack of drug-induced resistance. On the other hand, there is an emerging field of noncompetitive inhibitors targeting allosteric interactions between proteasomal subunits and offering a great potential of precise interventions into the cellular physiology. One of such inhibitors, a natural antibacterial peptide PR-39, has been shown recently to specifically block activation of a major transcription factor, NFκB, by the proteasome, and degradation of a regulator of oxygen distribution, HIF-1α, without affecting a gross intracellular protein catabolism. PR-39 and its derivatives are promising antiinflammatory agents and regulators of angiogenesis. Rational design of competitive and allosteric effectors of the proteasome is the challenge pursued by combined efforts of chemists and biologists.