The leukotriene biosynthetic pathway: a target for pharmacological attack

Iron chelating agents with clinical potential

Iron is a critically important metal for a wide variety of cellular events. The element holds this central position by virtue of its facile redox chemistry and the high affinity of both redox states (iron II and iron III) for oxygen. These same properties also render iron toxic when levels exceed the normal binding capacity of the cell. As a result of this potential toxicity, selective iron chelators are finding an important role in the treatment of iron overload associated with many forms of thalassaemia. In addition, they appear to have potential in treating situations where a local increase in iron concentration causes an unfavourable pathology, for instance, in reperfused tissue (heart disease and stroke) and in Parkinsonian brain. There is also evidence that iron chelators may minimise the toxicity of paraquat and the side effects of bleomycin and doxorubicin.

Non-haem iron enzymes can also be inhibited by iron chelators and consequently such enzymes as ribonucleotide reductase and lipoxygenase can be selectively inhibited. Such inhibitory action is being investigated for the treatment of malaria, neoplastic disease, psoriasis and asthma.

Recent developments in these areas are discussed in the present overview.

Derivatives of (Phenylsulfanyl)benzoic Acids with Multiple Antileukotrienic Activity

A series of derivatives of (phenylsulfanyl)benzoic acids bearing quinoline, 2,4-dihydroxy-3-propylacetophenone and 2,4-difluorobiphenyl moieties were prepared and their antileukotrienic activities evaluated. Some of the compounds were found to display multiple antileukotrienic effect in the inhibition of LTB4biosynthesis, binding to LTD4and LTB4receptors, superior to the standards (zileuton and zafirlukast) used. The compounds had an antiinflammatory effect, manifested with quinoline derivatives by a significant inhibition of bronchospasm induced by LTD4and/or albumin. The results of regression analysis correspond to the observation that the most active compounds belong to quinoline derivatives with the lowest lipophilicity. X-ray analysis of the quinoline compounds revealed that an intramolecular hydrophobic interaction of their aromatic rings does not occur in the solid state.

Inhibitors of the lipoxygenase pathway block development of Brugia malayi L3 in vitro

Brugia malayi L3 molt to the L4 stage in serum-free cultures supplemented with arachidonic, linoleic, or linolenic acids and the basidiomycetous yeast Rhodotorula minuta. These fatty acids are capable of entering the eicosanoid pathway of arachidonate metabolism, the pathway responsible for generating a number of biologically active mediators, including prostaglandins, leukotrienes, and lipoxins. To determine whether this pathway was required for L3 development, we added dual inhibitors of cyclooxygenase and lipoxygenase to in vitro cultures containing B. malayi L3. These compounds significantly inhibited L3 molting. To evaluate whether 1 or both of these pathways of arachidonate metabolism were involved in molting, we tested drugs inhibiting either cyclooxygenase or lipoxygenase. Lipoxygenase inhibitors blocked L3 molting, whereas cyclooxygenase inhibitors did not. To assess whether enzymes operating downstream of lipoxygenase were also involved in L3 molting, we added inhibitors of enzymes involved in leukotriene synthesis and found they were also capable of preventing development. We tested the same inhibitor panel on Dirofilaria immitis L3. A single lipoxygenase inhibitor and inhibitors of 2 different enzymes operating downstream of lipoxygenase disrupted D. immitis development. These results demonstrate that a lipoxygenase pathway product is required for molting of the infective stage larvae of filarial parasites.

The inhibition of [3H]leukotriene D4 binding to guinea-pig lung membranes. The correlation of binding affinity with activity on the guinea-pig ileum

Guinea-pig lung membranes contain high affinity (KD = 0.8 nM) binding sites for [3H]leukotriene D4 [( 3H]LTD4). The binding is inhibited by leukotriene antagonists, such as ICI 198615 and SK&F 104353, in a manner consistent with the Law of Mass Action at a single site. It is also inhibited by a range of leukotriene analogues in a dose-related manner. Inhibition by some of these e.g. LTC4 suggests that the [3H]LTD4 binding sites are heterogeneous. The binding affinity of the leukotriene analogues is significantly correlated (P less than 0.001) to their spasmogenic activity on guinea-pig ileum but not on guinea-pig lung strip. The binding affinity of the leukotriene antagonists is also correlated to their antagonist activity on guinea-pig ileum (P less than 0.05) but not on guinea-pig lung. These results indicate that the [3H]LTD4 binding site in guinea-pig lung is similar to the leukotriene receptor activated by LTD4 and LTE4 on guinea-pig ileum. The contractile response of guinea-pig lung strips to leukotrienes, in the presence of indomethacin, is mediated by a distinct type of leukotriene receptor.

Schistosoma mansoni: a comparison of secreted vs nonsecreted eicosanoids in developing schistosomulae and adults

We have determined that developing schistosomulae and adults of Schistosoma mansoni synthesize a wide range of eicosanoids when stimulated with linoleic acid, an essential fatty acid. Developing schistosomulae secrete 64%, while adults secrete over 80% of synthesized eicosanoids. On a per milligram soluble protein basis, eicosanoid secretion is ordered as follows: adult females greater than adult males much much greater than developing schistosomulae. Together one mature adult worm pair secreted approximately 4.36 micrograms prostaglandin E, 3.41 micrograms leukotriene B4, and 15.13 micrograms 5-hydroxyeicosatetraenoic acid (HETE) as determined by radioimmunoassay (RIA). High-performance liquid chromatography (HPLC) results have determined that 15-HETE is the major HETE species secreted by adults and developing schistosomulae. The immunosuppressant roles of 15-HETE, PGE, and LTB4 are discussed in relation to a possible mechanism for S. mansoni to evade the host immune system. Adults and schistosomules of S. mansoni have evolved rather sophisticated mechanisms for evading the host immune response. These include both host antigen acquisition and antigen shedding. In addition, young schistosomes have an as yet unidentified intrinsic defense mechanism against the host immune system. We postulate that part of the defense mechanism in schistosomules and adults may involve secretion of immunosuppressant eicosanoid species.

The binding of [3H]leukotriene C4 to guinea-pig lung membranes. The lack of correlation of LTC4 functional activity with binding affinity

High affinity binding sites for [3H]leukotriene C4 ([3H]LTC4) have been identified and characterised in guinea-pig lung membranes. [3H]LTC4 bound to these membranes with a pharmacological specificity totally distinct to that previously observed for [3H]LTD4 binding in guinea-pig lung. Scatchard analysis of saturation binding data showed a single class of binding sites, with a dissociation constant (KD) of 52.6 +/- 4.9 nM and a density (Bmax) of 30 +/- 12 pmol/mg membrane protein. The binding was inhibited with high affinity by a variety of glutathione-containing leukotriene analogues. Most notable was the inhibition by 1,2,3,4-tetranor LTC4 (Ki = 118 nM) and S-decylglutathione (Ki = 154 nM) since neither of these compounds were contractile agonists on guinea-pig parenchymal strips or guinea-pig ileum nor were they antagonists of LTC4-induced contractions of these smooth muscle preparations. These results indicate that the observed binding of [3H]LTC4 to guinea-pig lung membranes is not to a contractile receptor.

Oral nafazatrom in man: effect on inhaled antigen challenge

The effect of oral nafazatrom (Bay g6575, 2 X 3 g) or placebo on inhaled antigen challenge was assessed in a double-blind study. In four subjects antigen challenge resulted in an immediate fall of 93.2 +/- 3.36% in airflow at 40% of vital capacity (Vp40) and a 45.85 +/- 4.95% reduction in forced partial expiratory volume at one second (FEV1). Neither nafazatrom nor placebo had any effect on baseline lung function or that after challenge. Leukotriene B4 was generated by ex vivo stimulus of blood with ionophore A23187, and quantified by high performance liquid chromatography (h.p.l.c.)-radioimmunoassay. No inhibition of LTB4 formation occurred ex vivo following oral nafazatrom, although addition of 10(-5) M nafazatrom to blood in vitro significantly inhibited LTB4 release. Peak plasma nafazatrom levels during the study ranged from 3.3 X 10(-7) M to 1.47 X 10(-6) M which are below the concentration (10(-5) M) at which significant 5-lipoxygenase inhibition occurs in vitro. Oral nafazatrom is ineffective as a 5-lipoxygenase inhibitor in man, probably because of poor bioavailability after administration.