Table.

Gene therapy agents in recent development for Homozygous Familial Hypercholesterolemia

Agent	Stage of development	Efficacy data	Safety data
Recombinant LDLR-expressing AAV8 (AAV8.TBG.hLDLR)	Phase I/2a first-in-human study (NCT02651675). 12 patients. Primary safety endpoint at 1 year. Total follow-up of 5 years.	In LdLr−/−/Apobec1−/− mice maintained on a high-fat diet for two months prior to the administration of AAV8.TBG.mLDLR and then followed and analysed after two further months on this diet, TC was reduced by ~1500mg/dL and atherosclerotic burden by 87% [35]. Lipid lowering effects were maintained for up to 6 months post-vector administration in a cohort of LdLr−/−/Apobec1−/− chow-fed mice. Comparable findings in LdLr−/−/Apobec1−/−/hApoB-Tg mice treated with AAV8.TBG.hLDLR in [36]	In LdLr−/−/Apobec1−/− mice that received AAV8.TBG.mLDLR, hepatic lipid accumulation measured at day 35 and transaminases measured at day 60 post-vector administration were within normal limits [35]. In Ldlr−/−/Apobec1−/−/hApoB-Tg given the highest deliverable dose of AAV8.TBG.hLDLR (5×1013 GC/kg), no transgene or capsid-specific T cell responses were detected. There was also no significant rise in transaminases and no hepatic inflammation or lipid accumulation in AAV8.TBG.hLDLR-treated mice [36].
Gain-of-function LDLR variants (delivered within recombinant AAV8 vectors): AAV8.TBG.hLDLR-L318D (confers PCSK9 resistance)* AAV8.TBG.hLDLR -K809R\C818A (confers IDOL resistance)* AAV8.TBG.hLDLR -L318D\K809R\C818A (confers resistance to PCSK9 and IDOL)*	Pre-clinical (in vivo)	Humanized HoFH mice that received AAV8.TBG.hLDLR-L318D, AAV8.TBG.hLDLR-K809R\C818A or AAV8.TBG.hLDLR-L318D\K809R\C818A had lower TC levels and higher hepatic LDLR expression in the context of IDOL or PCKS9 over-expression compared to those that received the wild-type LDLR transgene [53].	N/A
HMGCR-specific miRNA/siRNA	Pre-clinical (in vivo)	Co-transfecting a plasmid containing a reporter gene driven by LDLR genomic regulatory elements with HMGCR-specific siRNA or miRNA increased LDLR promoter activity 300-fold and 12-fold, after 7 and 10 days, respectively, in vitro. In vivo, co-transfecting 4 HMGCR-specific miRNA plasmids with an LDLR-expressing plasmid with LDLR regulatory elements lowered plasma LDL-C by ~1.5mmol/L (p =0.0036) compared to mice treated with the LDLR plasmid alone [55].	N/A
Genetically corrected iPSC	Pre-clinical (in vitro)	Transfection of LDLR-expressing plasmids into iPSCs derived from patients with HoFH generated hepatocyte-like cells with restored LDL internalisation under feedback control from extracellular cholesterol, statin and sterols [58,59].	N/A

^*All three agents are recombinant AAV8 vectors expressing gain-of-function hLDLR variants resistant to PCSK9/IDOL or both. PCSK9 and IDOL are negative regulators of LDLR.

Key; Apobec, apolipoprotein B mRNA editing catalytic polypeptide-1; gc, gene copies; hLDLR, human LDLR; HMGCR, HMG CoA reductase.; IDOL, inducible degrader of LDLR; iPSC, induced pluripotent stem cell; miRNA, microRNA; TC, total cholesterol; siRNA, small interfering RNA oligonucloetides