Oral Iptacopan Monotherapy in Paroxysmal Nocturnal Hemoglobinuria

BACKGROUND

Persistent hemolytic anemia and a lack of oral treatments are challenges for patients with paroxysmal nocturnal hemoglobinuria who have received anti-C5 therapy or have not received complement inhibitors. Iptacopan, a first-in-class oral factor B inhibitor, has been shown to improve hemoglobin levels in these patients.

METHODS

In two phase 3 trials, we assessed iptacopan monotherapy over a 24-week period in patients with hemoglobin levels of less than 10 g per deciliter. In the first, anti-C5-treated patients were randomly assigned to switch to iptacopan or to continue anti-C5 therapy. In the second, single-group trial, patients who had not received complement inhibitors and who had lactate dehydrogenase (LDH) levels more than 1.5 times the upper limit of the normal range received iptacopan. The two primary end points in the first trial were an increase in the hemoglobin level of at least 2 g per deciliter from baseline and a hemoglobin level of at least 12 g per deciliter, each without red-cell transfusion; the primary end point for the second trial was an increase in hemoglobin level of at least 2 g per deciliter from baseline without red-cell transfusion.

RESULTS

In the first trial, 51 of the 60 patients who received iptacopan had an increase in the hemoglobin level of at least 2 g per deciliter from baseline, and 42 had a hemoglobin level of at least 12 g per deciliter, each without transfusion; none of the 35 anti-C5-treated patients attained the end-point levels. In the second trial, 31 of 33 patients had an increase in the hemoglobin level of at least 2 g per deciliter from baseline without red-cell transfusion. In the first trial, 59 of the 62 patients who received iptacopan and 14 of the 35 anti-C5-treated patients did not require or receive transfusion; in the second trial, no patients required or received transfusion. Treatment with iptacopan increased hemoglobin levels, reduced fatigue, reduced reticulocyte and bilirubin levels, and resulted in mean LDH levels that were less than 1.5 times the upper limit of the normal range. Headache was the most frequent adverse event with iptacopan.

CONCLUSIONS

Iptacopan treatment improved hematologic and clinical outcomes in anti-C5-treated patients with persistent anemia - in whom iptacopan showed superiority to anti-C5 therapy - and in patients who had not received complement inhibitors. (Funded by Novartis; APPLY-PNH ClinicalTrials.gov number, NCT04558918; APPOINT-PNH ClinicalTrials.gov number, NCT04820530.).

SAR443809: a selective inhibitor of the complement alternative pathway, targeting complement factor Bb

Dysregulated activation of the complement system is implicated in the onset or progression of several diseases. Most clinical-stage complement inhibitors target the inactive complement proteins present at high concentrations in plasma, which increases target-mediated drug disposition and necessitates high drug levels to sustain therapeutic inhibition. Furthermore, many efforts are aimed at inhibiting only terminal pathway activity, which leaves opsonin-mediated effector functions intact. We describe the discovery of SAR443809, a specific inhibitor of the alternative pathway C3/C5 convertase (C3bBb). SAR443809 selectively binds to the activated form of factor B (factor Bb) and inhibits alternative pathway activity by blocking the cleavage of C3, leaving the initiation of classical and lectin complement pathways unaffected. Ex vivo experiments with patient-derived paroxysmal nocturnal hemoglobinuria erythrocytes show that, although terminal pathway inhibition via C5 blockade can effectively inhibit hemolysis, proximal complement inhibition with SAR443809 inhibits both hemolysis and C3b deposition, abrogating the propensity for extravascular hemolysis. Finally, intravenous and subcutaneous administration of the antibody in nonhuman primates demonstrated sustained inhibition of complement activity for several weeks after injection. Overall, SAR443809 shows strong potential for treatment of alternative pathway-mediated disorders.

The alternative pathway of complement is activated in the glomeruli and tubulointerstitium of mice with adriamycin nephropathy

The complement system effectively identifies and clears invasive pathogens as well as injured host cells. Uncontrolled complement activation can also contribute to tissue injury, however, and inhibition of this system may ameliorate many types of inflammatory injury. Several studies have demonstrated that the filtration of complement proteins into the renal tubules, as occurs during proteinuric renal disease, causes tubular inflammation and injury. In the present study, we tested the hypothesis that activation of the complement system in the urinary space requires an intact alternative pathway. Using a model of adriamycin-induced renal injury, which induces injury resembling focal segmental glomerulosclerosis, we examined whether mice deficient in factor B would be protected from the development of progressive tubulointerstitial injury. Complement activation was attenuated in the glomeruli and tubulointerstitium of mice with congenital deficiency of factor B (fB-/-) compared with wild-type controls, demonstrating that complement activation does occur through the alternative pathway. Deficiency in factor B did not significantly protect the mice from tubulointerstitial injury. However, treatment of wild-type mice with an inhibitory monoclonal antibody to factor B did delay the development of renal failure. These results demonstrate that complement activation in this nonimmune complex-mediated model of progressive renal disease requires an intact alternative pathway.

Complement Activation via Alternative Pathway Is Critical in the Development of Laser-Induced Choroidal Neovascularization: Role of Factor B and Factor H

The objective of this study was to explore the role of classical, lectin, and alternative pathways of complement activation in laser-induced choroidal neovascularization (CNV). The classical and alternative pathways were blocked in C57BL/6 mice by small interfering RNAs (siRNA) directed against C1q and factor B, respectively. C4(-/-) mice developed CNV similar to their wild-type controls and inhibition of C1q by siRNA had no effect on the development of CNV. In contrast, CNV was significantly inhibited (p < 0.001) in C5(-/-) mice and C57BL/6 mice treated with factor B siRNA. Inhibition of the alternative pathway by factor B siRNA resulted in decreased levels of membrane attack complex and angiogenic factors-vascular endothelial growth factor and TGF-beta2. Furthermore, factor B was up-regulated in complement sufficient C57BL/6 mice at day 1 postlaser and remained elevated at day 7. Significantly reduced levels of factor H were observed at day 3 in these animals. In conclusion, our results demonstrate that activation of the factor B-dependent alternative pathway, but not the classical or lectin pathways, was essential for the development of CNV in mouse model of laser-induced CNV. Thus, specific blockade of the alternative pathway may represent a therapeutically relevant strategy for the inhibition of CNV.

Selective and efficient inhibition of the alternative pathway of complement by a mAb that recognizes C3b/iC3b

The alternative pathway (AP) of the complement system plays an important role in tissue damage and inflammation associated with certain autoimmune diseases and with ischemia-reperfusion injury. Selective inhibition of the AP could prevent such pathologies while allowing the classical and lectin pathways of complement activation to continue to provide protection. Here we present data describing selective inhibition of the AP of complement by anti-C3b/iC3b monoclonal antibody (mAb) 3E7, and by a chimeric, "deimmunized" form of this mAb, H17, which contains the human IgG1 Fc region and was further modified by substitution of amino acids in order to remove T cell epitopes. Both mAbs block AP-mediated deposition of C3b onto zymosan or Sepharose 4B, and they also inhibit AP-promoted lysis of rabbit erythrocytes. MAbs 3E7 and H17 also successfully compete with both factors B and H for binding to C3b-opsonized substrates, and the ability of both mAbs to inhibit the AP is blocked by pre-incubation with two different sources of C3(H2O). Kinetic measurements demonstrate that mAb 3E7 effectively stops progression of C3b deposition after AP activation is initiated. Our results therefore suggest that these mAbs block activation of the AP by binding to both C3(H2O) and to C3b, and thus prevent binding and activation of factor B. Based on these and other observations, mAb H17 may find future use in therapeutic applications focused on selective inhibition of the AP.

A novel inhibitor of the alternative complement pathway prevents antiphospholipid antibody-induced pregnancy loss in mice

Studies in gene-targeted mice have demonstrated that factor B of the alternative complement pathway plays an important role in several disease models, but an exogenous inhibitor of factor B has not previously been available. We have developed an inhibitory monoclonal antibody directed against a critical epitope on mouse factor B and have tested it in a model of antiphospholipid (aPL) antibody (Ab)-induced fetal loss. Gene-targeted factor B-deficient mice (fB-/-) were injected with a fusion protein comprised of the second and third short consensus repeat (SCR) domains of mouse factor B linked to a mouse IgG1 Fc domain. Hybridomas were made from splenocytes of the immunized mouse. One mAb, designated 1379, produced an IgG1 antibody that inhibited alternative pathway activation in vitro and in vivo by preventing formation of the C3bBb complex. Strikingly, this mAb inhibited alternative pathway activation in serum from mice, rats, humans, monkeys, pigs and horses. Fab fragments made from this mAb also inhibited alternative pathway activation. Epitope mapping demonstrated that this antibody binds to factor B within the third SCR domain. When mAb 1379 was administered to mice that also received human IgG containing antiphospholipid antibodies, it provided significant protection from antiphospholipid antibody-induced complement activation and fetal loss. Thus, this mAb to factor B has broad species reactivity and effectively inhibits alternative pathway activation. The mAb protects mice in an in vivo model of antiphospholipid antibody syndrome, demonstrating the therapeutic potential for the inhibition of factor B in this disease.

Investigation of the mechanisms of anti-complement activity in Ixodes ricinus ticks

The feeding success of a tick upon a host depends on its ability to suppress host anti-tick responses which include activation of the complement system. We investigated the mechanism of inhibition of the alternative pathway of complement by salivary gland extract (SGE) of the ixodid tick species, Ixodes ricinus. SGE treatment strongly inhibited C3a generation and factor B cleavage in serum when rabbit erythrocytes were used as complement activator, but not when cobra venom factor (CVF) was used as an activator. SGE treatment strongly inhibited C3b deposition on rabbit erythrocytes, and the turnover of C3 (to C3b/iC3b) in serum. However, there was no significant effect upon the formation, stability or activity of C3 convertase (C3bBb) when formed from purified C3b, factor B and factor D. SGE treatment of isolated C3 resulted in a shift in mobility of the alpha-chain (by about 5 kDa). N-terminal sequencing of this species suggests that cleavage occurs at the C-terminus of the alpha-chain of C3. Consistent with this hypothesis, the modified alpha-chain was still a substrate for pre-formed convertase. The activity was specific for the alpha-chain of C3 but not of C3(H2O) nor the alpha'-chain of C3b. It is proposed that SGE-modified C3 does not participate in convertase formation, probably having a reduced affinity for factor B.

C3 activation is inhibited by analogs of compstatin but not by serine protease inhibitors or peptidyl alpha-ketoheterocycles

C3 convertase is a key enzyme in the complement cascade and is an attractive therapeutic target for drug design. Recent studies have demonstrated that this enzyme is inhibited by compstatin (Morikis, D. , Assa-Munt, N., Sahu, A., Lambris, J.D., 1998. Solution structure of Compstatin, a potent complement inhibitor. Protein Sci. (7) 619-627; Sahu, A., Kay, B.K., Lambris, J.D., 1996. Inhibition of human complement by a C3-binding peptide isolated from a phage-displayed random peptide library. J. Immunol. (157) 884-891), a 13 amino acid cyclic peptide that binds to C3. Since the enzyme exhibits some homology to serine proteases, substrate-based design could be another avenue for drug design. In this study, we confirm the activity of compstatin using different sources of enzyme and different assay systems. We also tested the activity of substituted compstatin analogs and compared the selectivity and toxicity of these compounds to peptidyl alpha-ketoheterocyclic compounds. Our work confirms the activity of compstatin in both alternative and classical complement pathways, describes 11 new active analogs of this cyclic peptide, and provides evidence for key segments of the peptide for activity. Compstatin and related active analogs showed little or no inhibition of clotting or key enzymes in the clotting cascade nor did they appear to have significant cytotoxicity. The characteristics of compstatin suggest that this peptide and its analogs could be attractive candidates for further clinical development. By contrast, known serine protease inhibitors, including peptidyl alpha-ketoheterocycles, did not inhibit C3 convertase illustrating the atypical nature of this enzyme.

Induced fit activation mechanism of the exceptionally specific serine protease, complement factor D

We have investigated the mechanism by which the complement protease, Factor D, achieves its high specificity for the cleavage of Factor B in complex with C3(H2O). Kinetic experiments showed that Factor B and C3(H2O) associate with a KD of >/=2.5 microM and that Factor D acts on this complex with a second-order rate constant of kcat/KM >/= 2 x 10(6) M-1 s-1, close to the rate of a diffusion-controlled reaction for proteins of this size. In contrast, Factor D, which is a member of the trypsin family of serine proteases, was 10(3)-10(4)-fold less active than trypsin toward both thioester and p-nitroanilide substrates containing an arginine at P1. Furthermore, peptides spanning the Factor B cleavage site were not detectably cleaved by Factor D (kcat/KM </= 0.5 M-1 s-1). These results imply that contacts between Factor D and the C3(H2O)B complex, outside the vicinity of the cleavage site in Factor B, generate >/=9 kcal/mol of binding energy to stabilize the transition state for reaction. In support of this, we demonstrate that chemical modification of Factor D at a single lysine residue that is distant from the active site abolishes the activity of the enzyme toward Factor B while not affecting activity toward small synthetic substrates. We propose that Factor D may exemplify a special case of the induced fit mechanism in which the requirement for conformational activation of the enzyme results in a substantial increase in substrate specificity.

Rapid Isolation of Endogenous Inhibitor of Factor B

An inhibitor of mouse factor B was purified from lysate of L cells by a human factor B-affinity column. The purified inhibitor was found to be homogeneous with a molecular weight of about 25,000 and identical to the 25K protein isolated from L cell-lysate by the coprecipitation method with anti mouse factor B. Hemolytic titration of the activity and PAGE-analysis of the inhibitor-factor B complex showed that 1 mol of inhibitor reacted with 1 mol of mouse factor B and inactivated its hemolytic activity.

Characterization of alternative pathway inhibition by a serum derived, low molecular weight complement inhibitor

Normal human serum contains an inhibitor of complement which is distinguished by its small size of 500 daltons, the low molecular weight inhibitor (LMWI). When LMWI was present during incubation of zymosan or cobra venom factor with serum, formation of complement reactive complexes was blocked as measured by failure of these mixtures to lyse susceptible erythrocytes from patients with paroxysmal nocturnal haemoglobinuria (PNH). Addition of LMWI to pre-formed complexes had no effect on their subsequent haemolytic activity. When dialysis was used to remove LMWI from reaction mixtures, it was shown that LMWI had not irreversibly altered any of the complement components. Purified components were used to demonstrate that LMWI prevented factor D activation of cobra venom factor-factor B complexes. LMWI also strongly inhibited binding of 125I-factor B to human erythrocytes bearing C3b and had little or nor effect on binding of 125I-factor H to the C3b bearing cells. Factor B binding to C3b was equally inhibited on normal and PNH erythrocytes. Thus, a dialysable fraction from normal human serum prevents activation of human complement by blocking formation, but not the activity of the C3/C5 convertase. These low molecular weight inhibitors result in inhibition of factor B binding to C3b and inhibition of factor D activation of C3bB complexes.

Endogenous inhibitors of factor B

proteins which were able to bind noncovalently with mouse factor B were found in cells that are nonsecretors of factor B such as mouse-established monocytic cells and L cells but not in peritoneal resident macrophages. These proteins were isolated from lysates of L cells and separated into four distinct proteins by preparative SDS-polyacrylamide gel electrophoresis, with molecular weights of 25K , 28K , 33K, and 35K . The individual proteins formed a complex with purified mouse factor B at a molecular ratio of 1: 1 and inhibited its hemolytic activity. Proteins 25K and 28K inhibited the hemolytic activity of an activated form of factor B combined with cobra venom factor as well as that of the native form. These inhibitors did not affect the hemolytic activity of the second component of complement in mouse serum. The inhibitory activity of the 25K protein was partially inhibited by antiserum raised against it in rabbits.

New synthetic inhibitor to the alternative complement pathway

The inhibitory effects of 6-amidino-2-naphthyl 4-guanidinobenzoate (FUT-175) on the activities of factor B, factor D and cobra venom factor (CVF) X Bb were examined. FUT-175 bound specifically to the Bb fragment of factor B or CVF X Bb. FUT-175 was a non-competitive inhibitor of the esterolysis of L-leucyl-L-alanyl-L-arginine naphthylester by factor B and CVF X Bb. FUT-175 also inhibited the haemolytic activity of factor B, the C3 convertase activity of CVF X Bb and the factor B-cleaving activity of factor D. The concentration of FUT-175 causing 50% inhibition of these activities was 10(-5) - 10(-4)M.

Inhibitory effect of C-reactive protein on alternative C pathway activation by liposomes and Streptococcus pneumoniae

C-reactive protein (CRP) is an acute phase serum protein found associated with damaged tissue at sites of inflammation. CRP bound to multivalent phosphocholine-containing or polycationic ligands activates C by the classical pathway. We have previously described liposomes of a particular lipid composition that are able both to activate the alternative pathway of C and to bind CRP. In addition many strains of Streptococcus pneumoniae activate the alternative pathway. We have shown CRP binding to these bacteria as well. Because antibody to an activating surface in many cases enhances alternative pathway activation, we tested CRP for a similar function in these systems. Our results indicate that, in contrast to antibody, CRP inhibits alternative pathway activation. This inhibition by CRP is apparently restricted to surfaces that bind CRP. Thus, CRP binding to membrane or bacterial surfaces can convert them from alternative pathway activation to classical pathway activation.

Three rat monoclonal antibodies to human C3

Three monoclonal antibodies to human C3 have been obtained from a fusion of the rat myeloma line Y3 Ag 1.2.3. with spleen cells from rats immunized against C3. One, from clone 4, reacts with an antigenic determinant in C3c showing the expected reactivity of the 'C' antigen of C3. The specificity of the other two monoclonal antibodies correspond less clearly with known C3 antigens. By agglutination analysis of complement coated cells the determinant reacting with clone 3 is present in C3d while that for clone 9 appears as a neoantigen on C3bi. In both cases the co-precipitation results are anomalous and more direct studies are needed to define the exact specificity. The possibility that internal sequence duplications in C3 may explain some anomalies is discussed. None of the monoclonal antibodies significantly inhibit C3 functions. The monoclonal antibodies have been found to have unusual properties in co-precipitation assays being able to diffuse through a precipitation line with which they react to react with a further line. One antibody is also able to react strongly with the anodal half of what appears as a single line with a polyclonal antiserum.

Anticomplementary activity of cornea, vitreous and chorioretina in rabbits

We wished to determine if inhibitors of the complement system are present in different rabbit ocular tissues. Soluble extracts of cornea, vitreous humor and chorioretina in different dilutions were incubated with normal human serum (as source of complement). The ability of the latter to lyse sheep erythrocytes sensitized with antisheep rabbit hemolysin was assessed. The cornea did not show any inhibition of complement. The vitreous humor exhibited very low inhibitory activity. The extracts of the uveo-retinal tissue contained heat-labile and heat-stable factors capable of cleaving C3 and factor B.

The induction of macrophage spreading by factor B of the properdin system

Unstimulated mouse peritoneal macrophages attached to a glass substratum responded to activated human factor B (Bb) of the properdin system but not to native factor B with rapid spreading and a concomitant increase in their apparent surface area. Excellent correlation of the distribution of Bb protein and cell-spreading activity was found upon purification of Bb by ion-exchange and molecular seive chromatography and alkaline polyacrylamide gel electrophoresis. 1.6 microgram of purified Bb was sufficient to induce spreading in 50% of 5 x 10(4) glass attached macrophages within 1-2 h at 37 degrees C. Treatment of Bb with di-isopropyl-fluorophosphate indicated that the intact catalytic site of the serine-proteinase Bb was required for the initiation of macrophage spreading. The involvement of factor B in the induction of rapid cell spreading could also be indirectly demonstrated in an autologous system in which F(ab')2 fragments of an antiserum to mouse B prevented mouse macrophages from spreading in response to complement-activated mouse serum. These experiments suggest a role for factor B and the alternative pathway of complement fixation in the localization of mononuclear phagocytes to areas of inflammation.

Modulation of the formation of the amplification convertase of complement, C3b, Bb, by native and commercial heparin

Native rat mast cell macromolecular heparin proteoglycan and commercial hog heparin glycosaminoglycan chains inhibit generation of the amplification convertase, C3b, Bb. The inhibitory action of heparin is not due to chelation of magnesium. Heparin is most active in inhibiting convertase formation on cellular intermediates formed with the lowest C3b input and developed with the highest B concentration, thereby suggesting the receptor site for B on C3b as the point of heparin action. This interpretation is consistent with the demonstration that heparin prevents B utilization during the fluid phase interaction of C3b, B, and D. Inhibition is observed also when C3b,Bb generation takes place on cellular intermediates in the presence of P or C3NeF, which yield stabilized forms of the convertase. 50 times the concentration of heparin required to inhibit convertase generation does not accelerate the decay of the unstabilized or the C3NeF-stabilized convertases and has only a modest effect on the P-stabilized convertase. An additional effect of heparin is to impair beta1H-mediated decay-dissociation of C3b,Bb. The concentration of native or commercial heparin which prevents convertase formation is in the same range as that required for the demonstration of its anti-coagulant and anti-thrombin III cofactor activities. The additional finding that this inhibitory action of heparin can be expressed by the isolated mast cell granule suggests that native heparin may contribute to the modulation of the amplification pathway of complement.