SAN DIEGO – according to interim follow-up data from a phase 1/2 dose-escalation study.
The therapy – a self-complementary adeno-associated virus vector containing a codon-optimized factor IX gene, under control of a synthetic liver specific promoter and pseudotyped with serotype 8 capsid (scAAV2/8-LP1-hFIXco) – was previously shown to result in a dose-dependent increase in plasma factor IX levels in all 10 patients enrolled in the study, and an earlier update showed stable factor IX activity for at least 3 years, Ulrike M. Reiss, MD, reported at the annual meeting of the American Society of Hematology.
However, declining factor IX expression over time remains a concern, because AAV-mediated transgene expression is mediated mainly by episomally retained viral genomes, which may be lost with natural turnover of hepatocytes, noted Dr. Reiss, director of the clinical hematology division and the Hemophilia Treatment Center at St. Jude Children’s Research Hospital in Memphis.
At the “halfway mark,” with a median follow-up of 6.7 years in 10 patients aged 18-64 years who were treated with doses of either 2 x 1011, 6 x 1011, or 2 x 1012 vector genomes per kg (in 2, 2, and 6 patients, respectively), “factor IX expression has been persistent and stable in all participants after vector infusion,” she said.
“Factor IX expression was vector-dose dependent, achieving average levels of 1.9%-2.3% at the lower doses, and 5.1% at the high vector dose. All patients converted from having severe hemophilia to mild-moderate hemophilia,” she added.
The single significant adverse event observed during annual follow-up evaluations in the patients was a vector-related, immune-mediated liver inflammation occurring within 2-3 months of infusion in four of the six high-dose participants.
“There was complete resolution in all cases after a short course of corticosteroids over 8-12 weeks, including the taper. There were no late sequelae or any recurrence of transaminitis over time,” Dr. Reiss said. “We did not observe any new factor IX inhibitor or any late toxicity in any of these participants.”
Additionally, a comparison of average data across 3 years prior to gene therapy with the average data at 6.7 years after gene therapy showed that the annualized bleed rate decreased by 82% in the 10 participants and factor IX use decreased by 66%. In the high-dose group, the bleed rate decreased from 21 bleeds to 2 bleeds per year, and vector consumption was markedly reduced to a mean of 500 IU/kg per year from a mean of more than 2800 IU/kg per year. “Only one of the six patients in the high-dose group currently continues on prophylaxis treatment, whereas three in the low- and mid-dose groups are currently on prophylaxis,” she said. “In all [patients], the interval between prophylactic infusions has lengthened.”
Of note, Dr. Reiss and her colleagues explored the ability of using a modified, empty capsid-reduced vector preparation of the gene therapy to prevent the transaminitis seen in the 2-3 months after infusion. A new clinical preparation of scAAV2/8-LP1-hFIXco was manufactured with most of the empty particles removed by cesium chloride density centrifugation, but this approach provided no benefit in that regard.
“This further supports the observation that the anticapsid immune response is vector-dose dependent,” she said.
Additionally, the pattern of humoral response to AAV8 capsid was consistent with the primary immune response in participants.
“High IgG antibody titers have persisted for over 6 years; this finding is important because it will preclude these patients from any retreatment with the same vector or even potentially alternative AAV vectors of other serotypes with cross-reactive antigenicity,” she said.
Dr. Reiss reported having no relevant disclosures
SOURCE: Reiss UM et al. ASH 2018, Abstract 491.
