Monoclonal antibodies and amyloid removal as a therapeutic strategy for cardiac amyloidosis

Abstract

Cardiac amyloidosis (CA) is an infiltrative disease caused by progressive deposition of amyloid fibres in the heart. The most common forms include immunoglobulin light-chain and transthyretin amyloidosis. Current therapies for CA either stabilize or block the production of amyloidogenic precursors, preventing further amyloid deposition. This approach, while reducing cell damage and disease progression, does not target pre-existing amyloid deposits. Conversely, amyloid removal might stimulate functional recovery of the affected organ, thus improving quality of life and survival. A therapeutic strategy based on monoclonal antibodies capable of selectively binding amyloid deposits and inducing their removal has recently been tested in various clinical trial, with promising results, and could represent a key treatment for CA in the near future.

Introduction

Cardiac amyloidosis (CA) is a progressive infiltrative disease resulting from the deposition of amyloid fibrils in the heart. The most common forms include immunoglobulin light-chain amyloidosis (AL) and transthyretin amyloidosis (ATTR), caused by the deposition of unstable immunoglobulin free light chains and transthyretin (TTR), a tetrameric transport protein produced by the liver. Two forms of ATTR amyloidosis are recognized: wild-type (ATTRwt), due to aggregation of native TTR, showing a prevalent cardiac involvement; and hereditary (ATTRv), caused by mutations in the TTR gene leading to familial polyneuropathy (ATTR-FAP) and cardiomyopathy or a combinations based on the specific mutation.^1,2

Cardiac amyloidosis has long been considered a rare disease, with few available therapies and limited life expectancy. The major determinant of outcome in amyloidosis is the extent of cardiac involvement, suggesting that its removal may increase quality of life and survival.³ However, currently available therapeutic approaches aim to halt or stabilize the production of amyloidogenic precursors, thus impairing further amyloid deposition. To date, the standard of care includes: (i) chemotherapy and autologous stem cell transplantation for AL amyloidosis and (ii) stabilizer agents and gene silencers for ATTR amyloidosis.^1,2 Even after deposition is stopped, amyloid progressively continues to impair tissue function. Therefore, therapies that directly address the clearance of toxic amyloid are needed to restore organ physiology. One possible approach consists of targeting amyloid deposits with specific monoclonal antibodies (mAbs), thus stimulating their removal through phagocytic cells.⁴

This review summarizes the current evidence regarding the use of mAbs in the treatment of CA.

Disease-modifying therapies currently available for cardiac amyloidosis

Light-chain amyloidosis

The aim of treatment for AL amyloidosis is to eradicate plasma cell clones that produce amyloidogenic light chains. Most therapeutic strategies are adapted from those used for multiple myeloma.

Chemotherapy associated with autologous stem cell transplantation (ASCT) provides the best long-term outcome. The standard approach involves high-dose melphalan (an alkylating agent) combined with ASCT. However, only 20% of the patients are eligible for ASCT, which is reserved for patients with good functional status and no severe renal and cardiac involvement.¹ For transplant-ineligible patients, the initial treatment is oral melphalan with dexamethasone. The two other common regimens are cyclophosphamide–bortezomib–dexamethasone (CyBorD) and bortezomib–melphalan–dexamethasone. Indeed, an improvement in AL amyloidosis therapy has been represented by the introduction of proteasome inhibitors such as bortezomib, which either as a single agent or in combination with alkylating agents and/or dexamethasone, reduces the production of amyloidogenic light chains from plasma cells.¹ Second-generation proteasome inhibitors, carfilzomib and ixazomib, have been associated with a haematological response in more than half of patients with refractory AL amyloidosis, further improving their survival. Furthermore, immunomodulating agents such as thalidomide, lenalidomide, and pomalidomide, which act by inhibiting IL-6 expression and activating pro-apoptotic pathways, constitute important strategies for chronic therapy, maintaining a long-term haematological response.¹

Transthyretin amyloidosis amyloidosis

For a long time, the only therapy available for ATTR amyloidosis was liver transplantation or combined liver–heart transplantation. However, accumulation of ATTR in pre-existing amyloid deposits can progress even after liver transplantation. Recently, new pharmacological therapies directed towards different points of the amylogenic cascade have entered clinical practice.

Some therapies inhibit TTR gene expression through siRNA or antisense oligonucleotides. Patisiran is an siRNA which targets hepatocytes and mediates cleavage of TTR-mRNA to prevent its expression, and it has been approved for clinical use in patients with ATTR-FAP.⁵ Moreover, patisiran and vutisiran (another anti-TTR siRNA) are currently undergoing Phase III clinical trial for patients with ATTR-related cardiomyopathy in APPOLLO B (NCT03997383) and HELIOS B (NCT04153149) studies, respectively. Inotersen is an antisense oligonucleotide that binds to TTR-mRNA, thereby promoting its degradation. Inotersen is approved for the treatment of patients with ATTR-FAP.⁵ A promising perspective is represented by the CRISPR (clustered regularly interspaced short palindromic repeats)–Cas9 gene editing system, which has been proven to efficiently delete the TTR gene in vivo, reducing its production in the liver. This method is currently being investigated in a Phase I trial (NCT04601051).

Transthyretin stabilizers include tafamidis, acoramidis (AG-10), and diflunisal. These drugs interact with the tiroxine-binding site of TTR, thus inhibiting tetramer dissociation, which is the rate-limiting step in amyloidogenesis. Tafamidis was the first disease-modifying drugs to be approved for the treatment of patients with ATTR-related cardiomyopathy, after showing a reduction in all-cause mortality and in cardiovascular hospitalizations in the Phase III trial ATTR-ACT.² AG-10 is being evaluated in a Phase III trial on patients with ATTR-related cardiomyopathy (ATTRibute-CM, NCT03860935).

Other potential approaches for the treatment of ATTR amyloidosis are the inhibition of oligomer aggregation by epigallocatechin-3-gallate and the use of doxycycline and tauroursodeoxycholic acid, which promote amyloid fibrils disaggregation and might be a cheaper and feasible option, currently under evaluation in a Phase III trial (NCT03481972).²

Monoclonal antibodies in the treatment of amyloidosis

In 1975, Kohler and Milstein demonstrated the possibility of producing mAbs with predetermined specificity and low toxicity. Recently, mAbs have gained increasing interest in the treatment of several pathologies, such as cancer, autoimmune, and infectious diseases.

In CA, mAbs may potentially be used to target misfolded amyloidogenic precursors, plasma cell clones, or amyloid fibrils, thus inducing amyloid removal through different mechanisms depending on the specific target. Among them, antibody-dependent cellular toxicity (ADCC), opsonization, and neutralization of the amyloid burden have been found. Studies suggest that mAbs against cardiac amyloid mainly activate immune response with subsequent clearance by phagocytic cells.⁴

Monoclonal antibodies in the treatment of light-chain amyloidosis

Monoclonal antibodies targeting plasma cell clone

Daratumumab is a human anti-CD38 IgG1κ mAb (Table 1). CD38 is a surface antigen expressed on plasma cells. Daratumumab induces direct apoptosis of plasma cells through ADCC and is the first therapeutic anti-CD38 mAb approved for the management of multiple myeloma.⁴ Daratumumab has shown promising efficacy both as monotherapy and as combination therapy in the treatment of AL amyloidosis. In the Phase III ANDROMEDA trial, designed to assess the tolerability of daratumumab in addition to CyBorD, the safety run-in cohort of 28 patients showed an overall response rate of 96% and a complete response of 36% during the 1-year follow-up. Cardiac response was observed in 53% of the 17 evaluable patients.⁶ An extension of this study confirmed these results, showing a higher haematologic response (92 vs. 77%) and significant higher rates of cardiac responses (42 vs. 22%) compared with the control group (CyBorD without daratumumab).⁷ Based on this results, in 2021 FDA granted accelerated approval to daratumumab in combination with CyBorD for the treatment of AL amyloidosis.⁸

Isatuximab is a chimeric IgG1κ mAb that binds to CD38. A Phase II study on isatuximab for patients with relapsing AL amyloidosis or refractory to conventional therapies reported haematologic complete response, very good partial response, and partial response in 3, 54, and 20% of the treated patients, respectively.⁹

Elotuzumab is a humanized IgG1κ mAb that targets the cell-surface glycoprotein CD2 subset 1 expressed on plasma cells. Similar to daratumumab, elotuzumab appears to predominantly act through ADCC. However, elotuzumab has a maximal effect when combined with other agents such as lenalidomide or bortezomib.^8,10 A Phase II trial (NCT03252600) is currently evaluating the maintenance of treatment with elotuzumab, lenalidomide, and dexamethasone with or without cyclophosphamide, in first-relapsed AL amyloidosis.

Monoclonal antibodies targeting amyloid deposits

Birtamimab (NEOD001) is a humanized IgG1 mAb that binds to an epitope derived from a cleavage site of serum amyloid protein A (an apolipoprotein and one of the main acute phase proteins synthesized by the liver) and a cryptic epitope on AL amyloid fibrils exposed during misfolding. A Phase I/II study confirmed that birtamimab is safe and well tolerated in patients with AL amyloidosis. A Phase IIb study (PRONTO) evaluated birtamimab in patients with cardiac dysfunction and previously treated AL amyloidosis. However, the study did not meet the primary endpoint (cardiac response) after 12 months of follow-up. Therefore, the planned Phase III trial (VITAL) was halted. In the PRONTO trial, only Mayo Stage IV patients showed a significant survival benefit after birtamimab treatment. Thereby, a Phase III study on this category of patients (AFFIRM-AL; NCT04973137) was started in August 2021 and is estimated to be completed by June 2024.⁴

The monoclonal IgG1 antibody anselamimab (CAEL-101) is the chimeric form of murine mAb 11-1F4, which binds to a conformational neo-epitope of misfolded light chains, thus triggering the activation of macrophages.⁴ A Phase Ia/b study which enrolled patients with relapsed or refractory AL amyloidosis found early and sustained organ response in 67% of evaluable patients which was associated with a significant improvement in global longitudinal strain, while the remaining patients showed stable heart disease.¹¹ A Phase II trial (NCT04304144) is currently evaluating safety of the combination of CAEL-101 with CyBorD and the recommended dose of CAEL-101. Two Phase III studies have started enrolling patients with AL amyloidosis and advanced cardiac involvement (NCT04512235 and NCT04504825).

Monoclonal antibodies in the treatment of transthyretin amyloidosis amyloidosis

PRX004 is a humanized mAb specific to ATTRv amyloidosis (Table 2). A phase I study (NCT03336580) was designed to evaluate the safety of PRX004; however, it was terminated early due to the COVID-19 pandemic. Among the seven patients with available cardiac involvement, the authors reported an improved global longitudinal strain. Furthermore, no treatment-related serious adverse events were observed.^4,12

NI301A is a human mAb that binds to the linear epitope WEPFA, which is only accessible on misfolded TTR and ATTR deposits, triggering phagocytosis of ATTR aggregates by human macrophages, thus accelerating fibril removal.¹² NI301A is currently undergoing a Phase I clinical trial (NCT04360434) in patients with ATTR-related cardiomyopathy.^4,12

Ab-A is a human IgG1 mAb that targets aggregated TTR, with beneficial effects in a murine model of ATTRwt. Ab-A, which binds with high affinity to ATTR fibrils, was able to induce significant removal of ATTR aggregates by antibody-dependent phagocytosis. The ability to bind ATTR fibrils has also been demonstrated in human heart tissue samples with ATTRwt amyloidosis.¹³

Pan-amyloid antibodies?

A novel perspective derives from the development of pan-amyloid removal (PAR) therapeutics based on molecules capable of selectively binding amyloid deposits in all types of amyloidosis and at all stages of the disease.

Monoclonal antibodies targeting serum amyloid P

The first attempts to develop therapies against all amyloid deposits have focused on serum amyloid P (SAP), a protein belonging to the pentraxine family present in all human amyloid deposits, as it binds with high affinity but reversibly to amyloid fibres, inhibiting proteolytic degradation.¹⁴ Miridesap is a small molecule capable of promoting hepatic clearance of circulating SAP, but is unable to induce the removal of amyloid fibrils from tissues. Therefore, the possibility of combining miridesap with anti-SAP mAbs was tested to specifically target the remaining SAP in the deposits, with the aim of destabilizing amyloid fibrils and causing their removal. Dezamizumab is a fully humanized monoclonal IgG1 anti-SAP antibody, capable of binding amyloid deposits and inducing a response from giant multinucleate cells.¹⁵ In a Phase I trial in patients with systemic amyloidosis, the combination miridesap/dezamizumab was well tolerated and was able to improve liver function and reduce the amyloid load evaluated on imaging (SAP scintigraphy and cardiac magnetic resonance) in patients who had received a sufficient dose of antibodies in relation to their initial amyloid load.¹⁵ Patients with cardiac involvement were excluded; however, in an extension of the same study, miridesap therapy was evaluated in six patients with CA, in the absence of evidence of a reduction in cardiac amyloid load.¹⁶ The combination miridesap/dezamizumab was subsequently tested in patients with CA in a Phase II study (NCT03044353), which was terminated prematurely due to an excess of adverse events (mainly skin rashes). Since then, the development of this therapeutic approach has been discontinued.

New therapies of pan-amyloid removal

AT-02 was derived from the fusion of the PAR p5R peptide with an IgG1 antibody. The peptide binds to all amyloid types and has been shown to release the antibody at the site of interest by promoting a phagocytic response to amyloid deposits both in vitro and in vivo in an AL amyloidosis mouse model.

AT-02 will soon be tested in a Phase I clinical trial (NCT05521022).

AT-03 was obtained by fusion of the SAP protein with a single chain of the Fc portion of an igG1 antibody, which can interact with macrophages by inducing fibril phagocytosis. AT-03 ability to bind AL and ATTR human amyloid deposits has been tested in vitro and in murine CA models [data presented at the 13th Congress of the International Society of Amyloidosis (ISA), 2022].

AT-04 is a protein formed by fusion of a PAR peptide to the Fc component of an Ab igG1. It has been shown that this molecule binds with high affinity to AL, ATTR, and cerebral amyloid, inducing a phagocytic response (data presented at the 13th ISA Congress, 2022).

Conclusion

Early treatment of CA, both AL and ATTR, can significantly improve prognosis. However, current therapies can only block the deposition of additional amyloid without removing pre-existing deposits. Monoclonal antibodies offer an alternative mechanism of action compared to standard pharmacological treatments, with the advantage of high selectivity towards amyloid. Monoclonal antibodies directed against plasma cell clones are promising; therefore, daratumumab has already been approved for use in clinical practice. Similarly, Phase II and III trials are currently evaluating the safety and effectiveness of mAbs directed against AL or ATTR amyloid deposits. The production of anti-SAP antibodies, which initially showed promising results, was later stopped because of the suboptimal safety profile. However, a similar approach has been used to develop novel pan-amyloid therapies that could potentially be applied to all types of amyloidosis. Generally, anti-amyloid mAbs are intended as a complementary and synergistic approach to the current therapies for CA. Future studies evaluating combination therapies with drugs that reduce circulating levels of the amylogenic precursor and others that accelerate the amyloid removal from tissues are highly warranted.

Funding

None declared.

Data availability

No new data were generated or analysed in support of this research.

References

Author notes