What You Need to Know About the ADME Characteristics of EMA/FDA Approved siRNA Drugs?

Since 2008, several small interfering RNA (siRNA) drugs have received approval. Understanding the study designs and data characteristics of these five EMA/FDA approved drugs (Patisiran, Givosiran, Lumasiran, Inclisiran, and Vutrisiran) in terms of absorption, distribution, metabolism, excretion (ADME), and Drug-Drug interaction (DDI) is crucial, as it provides a valuable reference for future studies of candidate siRNA drugs.

(1)   Absorption

Oligonucleotide drugs face challenges in achieving sufficient exposure to target tissues. Unlike most small molecule drugs, they are not easily absorbed in the gastrointestinal tract. To overcome this, intravenous (IV) infusion and subcutaneous (SC) injection are commonly used to ensure adequate exposure to target tissues.

Patisiran, encapsulated in LNPs, with a particle diameter of approximately 60–100 nm, shows higher liver exposure after IV administration compared to SC¹. Therefore, IV was chosen for preclinical and clinical testing. The other four siRNA drugs enter the bloodstream directly via IV at high concentrations, which can lead to ASGPR saturation and limit drug entry into hepatocytes. SC administration facilitates a sustained-release effect, allowing gradual drug uptake by hepatocytes to achieve adequate exposure.

Systemic exposure, measured by plasma C_max and AUC_0-t values, showed a roughly proportional increase with the dose, and no significant sex differences were observed after administration to male and female animals. Following SC absorption, the plasma pharmacokinetic (PK) profile exhibited a characteristic two-compartment model^2–6.

(2)   Distribution

In vitro studies, such as plasma protein binding (PPB) assays, quantify the degree of binding to plasma proteins and aid in understanding compound distribution. Drug concentration changes over time in different tissues and organs are typically acquired through tissue collection or in vivo ADME studies, which involve quantitative whole-body autoradiography (QWBA) with radiolabeled drugs. In vivo assessments of these approved siRNA drugs have been conducted through significant tissue collection following the administration of unlabeled compounds or via QWBA assays to evaluate their distribution in vivo. The data available on in vivo tissue concentrations for these marketed siRNA drugs confirm their strong liver targeting, with all five showing prominent liver exposure. Furthermore, QWBA assays indicate that these approved siRNA drugs are primarily distributed in the liver.

(3)   Metabolism

Oligonucleotide drugs are primarily metabolized by endonucleases and exonucleases in the liver⁷ and other tissues rather than phase I and II metabolizing enzymes. Approved siRNAs, which are structurally modified, are less likely to be metabolized in circulation. Typically, the majority of siRNAs are quickly taken up by the liver, with only small fractions distributed in other tissues. Subsequently, siRNAs become susceptible to metabolism by nucleases in both the liver and other tissues.

In vitro metabolism studies for the five EMA/FDA approved siRNA drugs typically involve liver S9 and serum/plasma stability assays. For in vivo metabolism studies, plasma and liver tissues obtained from in vivo PK studies are commonly used for metabolite profiling. Additionally, data indicate that the in vitro and in vivo metabolic profiles of EMA/FDA approved siRNA drugs exhibit similarities, with these drugs sharing common metabolic pathways and metabolites across different species, including humans.

(4)   Excretion

The excretion processes of the five EMA/FDA approved siRNA drugs were primarily evaluated through mass balance studies using radiolabeled and unlabeled compounds. Due to their polarity, hydrophilicity, and negative charges, oligonucleotide drugs are predominantly excreted via the urine. During blood circulation, both GalNAc-siRNA and LNP-siRNA are primarily eliminated through rapid uptake by the liver, followed by metabolism. The kidneys serve as the secondary clearance pathway.

Among the five marketed siRNA drugs, only Inclisiran underwent a dedicated clinical PK study in patients with renal impairment. As for the other four siRNA drugs, their impact on drug concentration and efficacy in patients with mild to moderate renal impairment was assessed through population pharmacokinetic (PK) studies⁸. The results showed that although Cmax and AUC increased in patients with renal impairment, it did not affect the efficacy of siRNA drugs. No dose adjustments were made in patients with mild or moderate renal impairment.

(5)   Drug-Drug Interaction (DDI)

While oligonucleotide drugs are not anticipated to function as substrates of CYP450 enzymes or transporters, they could theoretically impact the expression of these enzymes and transporters by influencing regulatory pathways, for instance, oligonucleotide-targeted mRNAs might be involved in transcription and translation during the upstream process of CYP enzyme formation, potentially influencing CYP enzyme expression. However, DDI test results for each approved siRNA drug demonstrate that, with the exception of Givosiran, none of the marketed siRNA drugs inhibited CYP enzymes or acted as substrates or inhibitors of transporters.

At WuXi AppTec DMPK, we have established PK study strategies and an integrated in vivo and in vitro test platform for oligonucleotide drugs, aimed at accelerating new oligonucleotide drug development projects.

If you want to learn more details about the ADME characteristics of EMA/FDA approved siRNA drugs, please read the article now.

References:

¹ Chen, S. et al. Development of lipid nanoparticle formulations of siRNA for hepatocyte gene silencing following subcutaneous administration. J. Control. Release 196, 106–112 (2014).

² Witzigmann, D. et al. Variable asialoglycoprotein receptor 1 expression in liver disease: Implications for therapeutic intervention. Hepatol. Res. 46, 686–696 (2016).

³ Springer, A. D. & Dowdy, S. F. GalNAc-siRNA Conjugates: Leading the Way for Delivery of RNAi Therapeutics. Nucleic Acid Ther. 28, 109–118 (2018).

⁴. European Medicines Agency. European Public Assessment Report: Onpattro (patisiran). 44, (2018).

⁵ Nick Kozauer, MD（Alnylam Pharmaceuticals, I. . Cross-Discipline Team Leader Review. Patisiran NDA (2016).

⁶. Alnylam. Givosiran NDA MULTI-DISCIPLINE REVIEW.

⁷ Andersson, P. & Den Besten, C. CHAPTER 20: Preclinical and Clinical Drug-metabolism, Pharmacokinetics and Safety of Therapeutic Oligonucleotides. RSC Drug Discov. Ser. 2019-January, 474–531 (2019).

⁸ Jing, X., Arya, V., Reynolds, K. S. & Rogers, H. Clinical Pharmacology of RNAi-based Therapeutics: A Summary Based On FDA-Approved Small-interfering RNAs. Drug Metab. Dispos. DMD-MR-2022-001107 (2022) doi:10.1124/dmd.122.001107.

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