Alpha-1 Antitrypsin Deficiency: A Wealth of New Treatments Now in Clinical Testing

One day in 1962 while in his laboratory, the Swedish medical biochemist Carl-Bertil Laurell noticed an absence of the normally intense electrophoretic alpha-1 protein band in the plasma of two hospitalized patients suffering from emphysema with no known predisposing risk factors.¹ He determined that the missing protein band was alpha-1 antitrypsin (AAT), at that time a plasma protease inhibitor of uncertain significance. “A subnormal serum content of a1-antitrypsin may be a manifestation of an inherited predisposition to emphysema,” he concluded.

While it is now known to have anti- inflammatory, immunomodulatory, anti- infective and tissue-repair properties, AAT performs a particularly important physiologic regulatory function: It neutralizes excess free elastase and proteinase 3 generated by activated lung neutrophils, thus preventing excessive degradation of lung elastin and collagen connective tissue.

Encoded by the SERPINA1 gene, the normal AAT “M” protease inhibitor (Pi) allele is synthesized by hepatocytes and secreted into the circulation. To date, however, more than 120 mutations of the SERPINA1 gene have been identified, the two most frequent being the “S” and “Z” allelic variants. The “Z” variant in particular encodes a highly defective AAT protein, which misfolds and aggregates as Z-AAT polymers that accumulate within the endoplasmic reticulum of hepatocytes, instead of being secreted into the circulation.

Individuals with the PiMS and PiMZ genotypes remain at very low risk of pulmonary disease, as they express sufficient functional serum AAT to counterbalance neutrophil elastase. Those with the PiSZ genotype and serum AAT levels below a certain threshold can develop chronic progressive lung disease. But most severely affected individuals have the PiZZ genotype, meaning they are homozygous for the PiZ allele. In these individuals, serum AAT levels are typically only about 15 percent of normal. Compounding this problem, the AAT-Z polymers that do manage to reach the circulation have only 10 to 20 percent of normal AAT inhibitory capacity.²

Unsurprisingly, individuals with the PiZZ genotype account for more than 95 percent of cases of clinically apparent AAT deficiency (AATD),³ manifesting clinically as emphysema with or without chronic obstructive pulmonary disease (COPD). Chronic entrapment of hepatotoxic AAT-Z protein often additionally leads to the development of liver fibrosis and cirrhosis. Other less common sequelae of severe AATD include hepatocellular carcinoma, necrotizing panniculitis, granulomatosis with polyangiitis and bronchiectasis.

Altogether, the prevalence of AATD is estimated to be between one per 3,000 to 5,000 people, translating to roughly 70,000 to 100,000 affected individuals in the United States.⁴ However, as many as 90 percent of these individuals remain undiagnosed, as this relatively rare cause of obstructive lung disease tends to be overlooked. It can take many years for an AATD diagnosis to finally be established by specific testing, meaning thousands of individuals with AATD and active, symptomatic lung disease are walking around undiagnosed and untreated.

Since  the  U.S.  Food  and  Drug Administration   (FDA)   approval of the first product in 1987, lifelong augmentation and maintenance therapy with AAT concentrates* purified from plasma collected from healthy donors has remained the only available treatment for adult AATD patients with clinical evidence of emphysema (Table 1).

Table 1. Currently Available Human Alpha-1 Antitrypsin*

^{*Identified as alpha-1 proteinase inhibitor in labeling for all licensed products}

A few clinical studies, most recently the double-blind, randomized RAPID trial, have documented a significant reduction in the annual rate of CT-measured lung density loss at total lung capacity in study subjects who received the standard 60 mg/kg weekly AAT augmentation regimen compared to those assigned to placebo.⁵ A subsequent meta-analysis of three randomized placebo- controlled trials confirmed that AAT augmentation therapy was able to slow the progression of emphysema when measured by CT density.⁶

Unfortunately,  however,  standard human AAT augmentation therapy for AATD is limited by a number of significant shortcomings:

• The requirement for weekly intravenous (IV) dosing may dissuade some patients from initiating or remaining compliant with the lifetime AAT augmentation therapy regimen.
• The limited supply of donor plasma could eventually preclude treatment for currently undiagnosed patients with severe disease, as well as those with less severe forms of AATD (i.e., PiMZ and PiSZ genotypes) and evident lung disease who may also benefit from augmentation
• The high cost of lifelong AAT augmentation therapy, recently estimated at about $82,000 annually,⁷ can potentially create access barriers for some patients.
• AAT therapy addresses only AATD- associated lung disease; it does not treat or prevent serious liver disease or other extrapulmonary disease manifestations.

These shortcomings are currently the targets of a diverse range of new investigational therapies (Table 2), which can be broadly categorized as follows: 1) alternative human AAT delivery forms, 2) recombinant AAT products and 3) agents that interfere with hepatic production of abnormal AAT mutants.

Table 2. Leading Investigational Treatments for Alpha-1 Antitrypsin Deficiency

Alternative AAT Delivery Routes

Grifols (AAT-SC 15%). A Phase I/II clinical trial initiated by Grifols in 2021 is currently evaluating the safety, tolerability and pharmacokinetics of two different subcutaneously administered doses of a concentrated plasma-derived AAT (pdAAT) formulation — 72 mg/ kg and 144 mg/kg — against standard 60 mg/kg and doubled 120 mg/kg doses of its IV Prolastin-C Liquid AAT product.⁸

This small 16-subject trial is exploring both the feasibility of substituting more convenient subcutaneous delivery and the tolerability of a two-fold higher dose, with the attendant potential to further slow the rate of chronic lung injury in severely affected AATD patients. Separately, Grifols is continuing to enroll subjects with severe AATD and emphysema in a Phase III trial evaluating standard 60 mg/kg weekly IV dosing of Prolastin-C against 120 mg/kg weekly IV dosing of an investigational modified process AAT product, dubbed Alpha-1 MP, with assessments of multiple pulmonary function and quality-of-life endpoints over a three-year period.⁹

Kamada (AAT for inhalation). An obvious conceptual appeal of direct delivery of AAT to the lung parenchyma by inhalation is avoidance of the need for IV sticks, but its potential efficacy through that delivery route is uncertain. Kamada, an Israeli biotechnology firm that developed the first licensed liquid human AAT formulation (Glassia), is currently enrolling a planned total of 220 adult AADT patients with moderate or severe measured airflow limitation at a single study site in the Netherlands.¹⁰ Subjects are being randomized to receive 80 mg/day of human AAT formulated for inhalation through a nebulizer, or daily inhalation of a nebulized sodium chloride placebo solution. The primary endpoint is forced expiratory volume in one second (FEV1) after 104 weeks of treatment.

Recombinant AAT

Inhibrx (INBRX-101; rhAAT-Fc). Inhibrx has developed a novel extended half-life recombinant human AAT protein that incorporates the same immunoglobulin Fc domain fusion approach that has been successfully applied to prolong the half-lives of other licensed protein therapeutics.** INBRX-101 has been additionally engineered to maximize the protein’s functional AAT activity in the lungs.

Findings from a completed multiple ascending dose Phase I study showed the expected accumulation of functional AAT levels and achievement of fully normal levels in severely deficient AATD patients after five to six consecutive doses. Inhibrx believes that the time interval between INBRX-101¹¹ dosing may be extended from the weekly frequency required with plasma-derived AAT (pdAAT) to as long as every three to four weeks, while maintaining circulating AAT levels in the normal range (Figure).

Inhibrx plans to initiate a Phase II clinical trial using functional AAT as a surrogate endpoint with the intent to submit for regulatory approval under FDA’s accelerated approval program.

Figure. Overlay of Predicted Mean Serum AAT Levels with lnhibrx’ lNBRX-101 (rhAAT-Fc) Dosing Every 3 Weeks, and Published Mean Serum AAT Levels with Once-Weekly pdAAT Dosing

^{Source: Inhlbrx, Inc.}

RNAi Drugs to Block Hepatotoxic Z-AAT Protein Production

Dicerna Pharmaceuticals (belcesiran; DCR-A1AT). Massachusetts-based Dicerna, a Novo Nordisk company, develops RNA interference (RNAi) drugs that are designed to silence the genes that produce defective or undesirable proteins. In a transgenic mouse model of AATD, subcutaneous injection of its RNAi drug dramatically reduced both expression and intrahepatic polymer load of the hepatotoxic Z-AAT protein, compared to control mice receiving a saline injection. Extended monthly dosing of belcesiran additionally prevented development of liver fibrosis and other liver pathology in these experimental PiZ mice. A dose-dependent knockdown in the circulating AAT protein level was similarly achieved using a targeted RNAi agent in nonhuman primates.¹²

Belcesiran is currently being evaluated in the company’s randomized, multidose, double-blind,  placebo-controlled Phase II ESTRELLA trial to assess its safety, tolerability, pharmacokinetics and pharmacodynamics in participants with a diagnosis of PiZZ-type alpha-1 antitrypsin deficiency-associated liver disease (AATLD).¹³

Arrowhead Pharmaceuticals/Takeda (fazirsiran; ARO-AAT). Arrowhead has developed its own investigational RNAi therapeutic to reduce Z-AAT accumulation in the liver. In mid-2022, Arrowhead reported results from a Phase II clinical study involving 16 participants with AATLD who were homozygous for the PiZZ mutation, who received subcutaneous fazirsiran on day one and week four, and then every 12 weeks. All subjects had reduced accumulation of Z-AAT in the liver, with a median drop of 83 percent at week 24 or 48. The nadir in serum was a reduction of approximately 90 percent. Associated with these findings was a reduction in histologic globule burden, from a mean score of 7.4 (on a 0 to 9 scale) at baseline to 2.3 at week 24 or 48. Fibrosis progression was observed in seven of 15 patients, while progression was observed in just two of 15 patients.¹⁴

In January of this year, Arrowhead announced topline results from its Phase II SEQUOIA clinical study:¹⁵ a median 94 percent reduction in serum mutant Z-AAT concentration at week 48 with the highest of three tested fazirsiran doses (200 mg), and a median 94 percent reduction in total liver Z-AAT at the postbaseline liver biopsy visit. PAS-D globule burden, a histological measure of Z-AAT accumulation, fell by 68 percent between baseline and the postbaseline liver biopsy visit. Half of patients at all dosage levels (25 mg, 100 mg and 200 mg) achieved a measurable improvement in fibrosis. In contrast, patients receiving placebo who had baseline fibrosis experienced no change in serum Z-AAT, a 26 percent increase in liver Z-AAT, and no improvement in PAS-D globule burden. Treatment-emergent adverse events were generally well-balanced between fazirsiran and placebo groups.

Arrowhead and Takeda signed a collaboration and licensing agreement in October 2020, under which they will continue to co-develop fazirsiran and, if approved, co-commercialize the product in the U.S.

Oral Neutrophil Elastase Inhibitors

Mereo BioPharma (MPH966; alvelestat). Alvelestat acts to inhibit the neutrophil elastase enzyme whose activity in healthy individuals is inhibited by normal endogenous AAT. Mereo believes that this orally administered drug has the potential to protect AATD patients from further lung damage. Recognizing it as a potential first-in-class oral neutrophil elastase inhibitor, FDA has granted alvelestat a fast track drug designation. Results from the United Kingdom- based company’s Phase II ASTRAEUS study of alvelestat in patients with severe AATD-associated emphysema demonstrated statistically significant changes relative to placebo in three primary biomarker endpoints associated with AATD-related lung disease, including up to 90 percent reduction in blood neutrophil elastase activity.¹⁶

In collaboration with the National Institutes of Health and investigators at the University of Alabama at Birmingham, Mereo is currently enrolling subjects with confirmed AAT in a Phase II randomized, double-blinded clinical trial in a total of 66 participants with confirmed AAT, who are being randomized to receive 120 mg of alvelestat or a placebo pill taken orally twice-daily for 12 weeks.¹⁷ The primary endpoint is within-individual percentage change in blood markers for neutrophil elastase activity. A readout of preliminary findings from this study are anticipated in mid-2023.

Small Molecule AAT Correctors

Vertex Pharmaceuticals (VX-634 and VX-864). Vertex has developed a new class of oral AATD-targeted drug candidates that could potentially restore the elastase inhibitor function of abnormal AAT, as well as resolve its hepatotoxicity. These investigational small molecule “AAT correctors” are designed to promote proper folding of Z-AAT protein in individuals with the PiZZ genotype, correcting it to become functional AAT (fAAT) that can be expressed by hepatocytes into the bloodstream.

A completed 28-day Phase II study showed that VX-864 treatment reduced levels of Z-AAT polymer in the blood of AATD patients by an average of 90 percent from baseline, with modest increases in fAAT. In December 2022, Vertex initiated a second Phase II clinical trial to examine whether longer- term VX-864 administration also results in liver Z-AAT polymer clearance and, if so, whether this longer-term treatment might also result in larger increases in plasma fAAT levels.¹⁸ A total of 20 participants will take VX-864 tablets orally every 12 hours for 48 weeks; the primary outcome measure is change in functional blood AAT levels from baseline.¹⁹

Additionally, Vertex is now evaluating the safety and tolerability of a new small molecule AAT corrector with significantly improved potency, dubbed VX-634, in more than 100 healthy volunteers.²⁰

At present, physicians have only intravenous AAT augmentation therapy to try to limit the unrelenting progression of lung damage in severely affected patients with the PiZZ, PiSZ and PiNull genotypes. And they have nothing to address fibrotic or cirrhotic disease sequelae.

But should a number of the investigational agents now in clinical testing prove safe and effective, one can envision a scenario where physicians can employ therapeutics in combination to address the imbalance in lung neutrophil protease activity to prevent further destructive damage, and to block the accumulation of abnormal AAT in hepatocytes to protect the liver. This scenario should additionally provide greater impetus for physicians to order AATD testing on all individuals with COPD and unexplained chronic liver disease, as well as relatives of individuals identified with an abnormal gene for AAT, as currently recommended by the Scientific Advisory Committee of the Alpha-1 Foundation.²¹

It is now conceivable that physicians could soon be empowered to effectively manage and even prevent overt lung and liver disease in their patients with AATD through early detection and timely initiation of treatments they select from an expanded armamentarium. The clinical trial findings that become available over the next several years could prove to be transformational for the many thousands of individuals diagnosed — and yet to be diagnosed — with AATD.