Small Interfering RNA (siRNA) Therapy

Mechanism of Action

The siRNAs are identified as small double-stranded RNAs that exert their effects by dividing into single strands and binding to their distinct target messenger RNA (mRNA) sequences. This action catalyzes a string of activities that yield the target mRNA to degrade, further halting translation and inducing gene suppression by the short RNA strands. This enzyme is also known as RNA interference (RNAi).[9][10]

The siRNA agents initiate their RNAi action in the cytoplasm with the help of an enzyme, endoribonuclease dicer, also known as helicase, with an RNase motif. Helicase cleaves the double-stranded RNA and yields a siRNA.

The short double-strand RNA fragments integrate with a cluster of essential proteins known as the RNA-induced silencing complex (RISC) and are then split into 2 single RNA strands recognized as sense and antisense. The single antisense strand further targets its mRNA arrangement while the sense strand is dismissed from the complex. The essential enzymes in the RISC complex, such as the Ago-2 endonucleases, cleave their target mRNA sequence once affixed.[9][10][11]

Patisiran: Patisiran exerts its mechanism of action through RNAi, resulting in cleavage and breakdown of all types (mutant and wild-type) of transthyretin (TTR) mRNA. The agent is a double-stranded siRNA formulated as a lipid nanoparticle (LNP) taken up by hepatocytes once bound to apolipoprotein E (APOE) receptors. The TTR mRNA's RNAi decreases the TTR protein's circulation production and deposition in tissues and organs.[12][13]

Givosiran: Givosiran exerts its mechanism of action through RNAi, resulting in the cleavage and breakdown of its target, aminolevulinate synthase 1 (ALAS1) mRNA, found in the liver. The agent is a double-stranded siRNA conjugated with N-acetylgalactosamine (GalNAc) ligand for hepatocyte intake. Once taken up by hepatocytes and degrading its target mRNA, it decreases aminolevulinic acid (ALA) and porphobilinogen (PBG) levels in the blood, further limiting AHP disease characteristics.[14][2]

Lumasiran: Lumasiran exerts its mechanism of action through RNAi in the liver, resulting in cleavage and breakdown of its target, hydroxy acid oxidase 1 (HAO1) mRNA. The agent is a double-stranded siRNA conjugated with GalNAc ligand for effective hepatocyte uptake. HAO1 produces glycolate oxidase (GO), an enzyme responsible for producing glyoxylate, a substrate for the further synthesis of oxalate. The inhibition of the GO enzyme results in decreased oxalate precursor levels, reducing the production of the enzyme alanine glyoxylate aminotransferase (AGT) that is mutated in PH1.[9][15][14]

Inclisiran: Inclisiran exerts its mechanism of action through RNAi in the liver and orchestrates hepatic mRNA degradation of proprotein convertase subtilisin/kexin type-9 (PCSK9). The sense strand of the siRNA is bound with the GalNAc ligand, allowing hepatocytes to uptake the siRNA agent and target PCSK9 efficiently. PCSK9 internalizes and breaks down the hepatic LDL receptors once attached. Inclisiran inhibits this action of PCSK9, further promoting the expression of LDL-C receptors on the cell's surface and facilitating receptor cycling. Once LDL-C is bound to its receptor, it is subject to degradation by lysosomal enzymes and recycled back to the cell's surface. This results in a raised uptake of LDL-C and reduces its levels in the blood.[16]

Nedosiran: Nedosiran, a double-stranded small interfering RNA (siRNA) conjugated to GalNAc aminosugar residues, is administered subcutaneously. Upon administration, the GalNAc-conjugated sugars bind to asialoglycoprotein receptors (ASGPR), facilitating nedosiran delivery to hepatocytes. The siRNA action of nedosiran leads to the degradation of hepatic lactate dehydrogenase (LDH) by targeting LDHA messenger ribonucleic acid (mRNA) through RNA interference. This reduction in hepatic LDH levels diminishes oxalate production by the liver, subsequently reducing the oxalate burden.[17]

Vutrisiran: Vutrisiran, a double-stranded small interfering RNA (siRNA) with a GalNAc conjugate, induces the degradation of mutant and wild-type transthyretin (TTR) mRNA through RNA interference. This mechanism leads to a decrease in serum TTR protein levels and the deposition of TTR protein in tissues.[7]

Administration

Available Dosage Forms, Strengths and Dosage

Patisiran

Patisiran is available and distributed as a single-dose vial of 10 mg/5 mL (2 mg/mL). A lipid complex injection should be filtered and diluted before administering. A healthcare professional should calculate and adjust the correct dosage according to the patient's body weight and administer it as an intervenous infusion.

Indication: polyneuropathy of hereditary transthyretin-mediated amyloidosis in adults.

• 0.3 mg/kg once every 3 weeks (weight <100 kg)

• 30 mg once every 3 weeks (weight ≥100 kg)

Patisiran should be administered immediately if an earlier dose was missed. Patients given a dose within 3 days from a missed dosage should follow their original regime. 

Patients receiving treatment with patisiran are advised to receive premedication therapy 60 minutes before administration of the IV infusion to decrease the occurrence of infusion-related reactions (IRRs). Premedication therapies consist of corticosteroids (IV), acetaminophen (oral), H1 blockers (IV), and H2 blockers (IV).

Givosiran

Givosiran is available and distributed as a single-dose vial of 189 mg/mL for subcutaneous injection.

Indication: acute hepatic porphyria in adults.

• 2.5 mg/kg once monthly

Givosiran should be administered immediately if a prior dosage was missed and continued monthly afterward. Patients with critical elevations in liver transaminases should decrease their dosage to 1.25 mg/kg every month. In subjects who do not show a recurring increase in transaminases while receiving therapy at 1.25 mg/kg, the dosage can be increased to 2.5 mg/kg.

Lumasiran

Lumasiran is available and distributed as a single-dose vial of 94.5 mg/0.5 mL for subcutaneous injection. Lumasiran is administered as loading doses accompanied by maintenance doses calculated and adjusted according to the patient's body weight.

Indication: PH1 adult and pediatric subjects.

• Initial loading dose: 6 mg/kg administered once a month for 3 doses (weight <10 kg)
• Maintenance dose: 3 mg/kg administered once a month (weight <10 kg)

• Initial loading dose: 6 mg/kg administered once a month for 3 doses (10 to 20 kg)
• Maintenance dose: 6 mg/kg administered once every 3 months quarterly (10 to 20 kg)

• Initial loading dose: 3 mg/kg administered once a month for 3 doses (≥20 kg)
• Maintenance dose: 3 mg/kg administered once every 3 months (≥20 kg)

Lumasiran should be administered immediately if a prior dosage was missed and continued monthly thereafter.

Inclisiran

Inclisiran is available and distributed as a pre-filled single-dose vial of 284 mg/1.5 mL (189 mg/mL) for subcutaneous injection.  

Indication: reducing LDL-C in subjects with HeFH or ASCVD adjunctive to diet and statin therapy tolerated at a maximum dose.

• 284 mg (initial dose)

• 284 mg (at 3 months)

• 284 mg (every 6 months after that)

Inclisiran should be administered immediately if a prior dosage was missed. Patients given a dosage within 3 months of a missed dosage are recommended to follow their original regime. If a dose has been missed for more than 3 months, therapy should be initiated at 3 months following administration of the missed dosage and every 6 months after that.

Nedosiran

Nedosiran is available in 128mg/0.8 mL as a single-dose pre-filled syringe and 160 mg/mL as a single-dose pre-filled syringe, both intended for subcutaneous injection.

Indication: Lower urinary oxalate levels are reduced in individuals aged 9 and older with primary hyperoxaluria type 1 (PH1) and preserved kidney function (eGFR ≥ 30 mL/min/1.73 m2).

• Adults and Adolescents (age 12 and older)

  • Greater than or equal to 50 kg: 160 mg once monthly (pre-filled syringe, 1 mL)
  • Less than 50 kg: 128 mg once monthly (pre-filled syringe, 0.8 mL)

• Children (ages 9 to 11)

  • Greater than or equal to 50 kg: 160 mg once monthly (pre-filled syringe, 1 mL)
  • Less than 50 kg: 3.3 mg/kg once monthly, not to exceed 128 mg (round to nearest 0.1 mL)

Vutrisiran

Vutisiran is available as a single-dose pre-filled syringe containing 25 mg/0.5 mL for subcutaneous injection.

Indication: Polyneuropathy of hereditary transthyretin-mediated amyloidosis in adults.

• Recommended Dosage: 25 mg administered by subcutaneous injection once every 3 months.

It is recommended that the recommended amount of vitamin A be supplemented daily while taking vutrisiran. If any ocular symptoms that suggest a vitamin A deficiency occur, it is recommended to consult an ophthalmologist.[7]

Adverse Effects

Adverse events are listed by the agent below.[16]

Patisiran

• Upper respiratory tract infections (URTI) (28%)
• Infusion-related reactions (19%)
• Dyspepsia (8%)
• Dyspnea (8%)
• Muscle spasms (8%)
• Arthralgia (7%)
• Erythema (7%)
• Bronchitis (7%)
• Vertigo (5%)

Givosiran

• Nausea (27%)
• Injection site reactions (25%)
• Rash (17%)
• Serum creatinine increase (15%)
• Elevated liver transaminases (13%)
• Fatigue (10%)

Lumasiran

• Injection site reactions (38%)
• Abdominal pain/discomfort (15%)

Inclisiran

• Arthralgia (4%)
• Urinary tract infection (3.6%) 
• Diarrhea (3.5%)
• Bronchitis (2.7%)
• Pain in extremity (2.6%)
• Dyspnea (2.6%)
• Injection site reaction (1.8%)

Nedosiran

• Injection site reactions (≥20%) [18]

Vutrisiran

• Pain in extremity (15%)
• Arthralgia (11%)
• Dyspnea (7%)
• Vitamin A deficiency (7%) [7]
• Injection site reactions (4%)
• Rare case report of atrioventricular block.[19]

Contraindications

No labeled contraindications have been reported for patisiran, lumasiran, neodosiran, vutisiran and inclisiran. Hypersensitivity to givosiran is an absolute contraindication to therapy.[20] Anaphylaxis has been reported in <1% of subjects during clinical trials with givosiran.[21]

Monitoring

Patients receiving therapy with patisiran should be monitored for infusion-related adverse responses such as hypersensitivity reactions and anaphylaxis. Treatment with patisiran should be delayed or interrupted, and appropriate management should be initiated as clinically required. Infusion should be resumed gradually once adverse manifestations are resolved. In the event of critical infusion-related adverse responses, therapy should not be continued. Patients receiving treatment with patisiran should also have their vitamin A levels monitored routinely as therapy decreases its levels. Patients should be initiated on vitamin A supplementation that does not exceed the recommended daily dosage and monitored for clinical manifestations of vitamin A deficiency.[22]

Liver function tests (LFTs) should be monitored at baseline and routinely after that in patients receiving treatment with givosiran, as an increase in alanine transaminase (ALT) was observed during clinical trials. Elevation in liver function is reported to occur around 3 to 5 months from the initiation of therapy. Recommendations are to measure LFTs monthly for the first 6 months of givosiran. Treatment should be discontinued in patients with severe elevations in ALT. Baseline and routine monitoring of estimated glomerular filtration rate (eGFR) and serum creatinine levels is recommended along with LFTs when receiving givosiran. Alterations in eGFR and creatinine were reported during clinical trials.

Patients receiving givosiran should also have their homocysteine levels measured at baseline and routinely afterward. Elevations in homocysteine levels in the blood were reported in 16% of subjects during clinical trials. Patients with evidence of homocysteine elevations should have their vitamin B9 and B12 levels assessed and consider initiating vitamin B6 supplementation.

Immunogenicity with all siRNA agents is possible and may be a barrier to therapeutic delivery. An unwanted biological immunogenic response may identify siRNA as a viral RNA antigen, provoking undesirable adverse reactions. During clinical trials, anti-drug antibodies (ADA) developed in 3.6% of subjects receiving patisiran, 6% of subjects receiving lumasiran, 0.9% of subjects receiving givosiran, 1.8% of subjects before inclisiran dosing, and 4.9% of subjects at 18 months therapy. ADA did not influence effectiveness or safety in all siRNA agents, resulting in no clinically significant discrepancies in the pharmacokinetics and pharmacodynamics.[23]

Toxicity

During clinical trials with givosiran, hepatic and renal toxicity was reported. Fifteen percent of subjects receiving therapy reported ALT elevations 3 times above the normal range. Fifteen percent of subjects reported elevated serum creatinine levels and reductions in eGFR, with a median elevation in creatinine of 0.07 mg/dL at 3 months. The severity of laboratory alterations and clinical symptoms should determine whether to continue therapy. 

Pregnancy

Data on using patisiran and lumasiran therapy during pregnancy has not been reported. A reduction in vitamin A levels is recognized with patisiran use, a crucial component of normal embryo maturation and fetal development. An excess in vitamin A levels correlates with adverse effects on the fetus. 

Although givosiran was not studied in humans during pregnancy, animal reproduction analyses demonstrated unfavorable developmental effects during organogenesis. Givosiran during pregnancy should be assessed, weighing the advantages and dangers to the mother and probable impacts on the developing fetus. Inclisiran therapy is recommended to be discontinued during pregnancy. This agent's mechanism of action has been reported to decrease cholesterol, and biologically active substances may harm the developing fetus.[24]

Enhancing Healthcare Team Outcomes

Small interfering ribonucleic acid (siRNA) based therapies have recently been approved as therapeutics exerting their effects by RNAi on their target mRNA in the liver. To date, 6 agents (patisiran, givosiran, lumasiran, inclisiran, nedosiran, and vutisiran) are FDA-approved for managing adult patients with hATTR, AHP, reducing LDL-C in subjects with HeFH or ASCVD, and PH1 in adults and pediatric patients. Caring for patients with rare metabolic ailments demands care coordination from a team of healthcare professionals. The healthcare team should include a primary care physician (PCP), a hematologist, a nephrologist, a cardiologist, a physician assistant, a nurse practitioner, a clinical geneticist, nurses, and a pharmacist.

The healthcare team should conduct a thorough clinical examination to assess if the management of siRNA-based therapies suits the patient. Before prompting treatment, the interprofessional team should determine the patient's laboratory values, such as renal and liver function (givosiran), lipid levels (inclisiran), and vitamin A levels (patisiran, vutisiran) at baseline and periodically. The PCP should regularly communicate with the other specialists caring for such patients.

The healthcare team should administer the abovementioned premedications (corticosteroid, acetaminophen, H1 blocker, and H2 blocker) at least 60 minutes before the infusion to prevent an infusion-related reaction. The prescribing and administering clinician should be prepared and equipped for immediate adverse effects such as infusion-related hypersensitivity reactions. The healthcare team should monitor for headaches, back pain, abdominal pain, shortness of breath, and nausea during infusion. In the event of clinical signs such as hypotension, syncope, and anaphylaxis, the infusion should be discontinued immediately, and appropriate symptomatic management should be initiated.

The interprofessional team should educate their patients on the probable adverse effects of siRNA-based therapies and counsel them regarding the appropriate measures. Vitamin A levels require routine monitoring by the prescribing healthcare provider for patients receiving patisiran and vutisiran, as the agents are reported to decrease their levels.

Childbearing females considering treatment with siRNA-based therapies during pregnancy should be evaluated to determine the benefits and risks before starting therapy. Although no studies and data involving pregnant women have been reported, pregnancy risks merit discussion. Patisiran reportedly decreases vitamin A levels, which is essential for fetal development. Excess vitamin A levels may also cause adverse developmental consequences. Pregnant patients considering givosiran should be informed about unfavorable fetal developmental outcomes during organogenesis in animal models. Inclisiran should be discontinued during pregnancy. Effective communication among the primary healthcare team and specialists treating patients with siRNA-based therapies can reduce complications and enhance quality of life and patient outcomes.[9][11]