The acute
hepatic porphyrias are caused by inherited enzymatic deficiencies in the
heme biosynthesis pathway. Induction of the first
enzyme 5-aminolevulinic
acid synthase 1 (ALAS1) by triggers such as fasting or
drug exposure can lead to accumulation of neurotoxic
heme intermediates that cause disease symptoms. We have demonstrated that hepatic ALAS1 silencing using
siRNA in a
lipid nanoparticle effectively prevents and treats induced attacks in a mouse model of
acute intermittent porphyria. Herein, we report the development of ALN-AS1, an investigational GalNAc-conjugated
RNAi therapeutic targeting ALAS1. One challenge in advancing ALN-AS1 to patients is the inability to detect liver ALAS1
mRNA in the absence of liver biopsies. We here describe a less invasive circulating extracellular
RNA detection assay to monitor RNAi
drug activity in serum and urine. A striking correlation in ALAS1
mRNA was observed across liver, serum, and urine in both rodents and nonhuman primates (NHPs) following treatment with ALN-AS1. Moreover, in donor-matched human urine and serum, we demonstrate a notable correspondence in ALAS1 levels, minimal interday assay variability, low interpatient variability from serial sample collections, and the ability to distinguish between healthy volunteers and
porphyria patients with induced ALAS1 levels. The collective data highlight the potential utility of this assay in the clinical development of ALN-AS1, and in broadening our understanding of acute
hepatic porphyrias disease pathophysiology.