Baseline Correlates of Complete Response to Idecabtagene Vicleucel (ide-cel, bb2121), a BCMA-Directed CAR T Cell Therapy in Patients with Relapsed and Refractory Multiple Myeloma: Subanalysis of the KarMMa Trial

Introduction Outcomes for patients with relapsed and refractory multiple myeloma (RRMM) and previous exposure to immunomodulatory agents, proteasome inhibitors (PIs), and anti-CD38 antibodies are poor. Ide-cel is a B-cell maturation antigen (BCMA)-directed CAR T cell therapy. In the pivotal phase 2 KarMMa trial (NCT03361748), ide-cel demonstrated frequent, deep, and durable responses in heavily pretreated patients with RRMM (Munshi et al. N Engl J Med 2021). In the 42 patients (33%) who achieved compete response (CR) or stringent CR (sCR), median duration of response was 21.5 months (Anderson et al. ASCO 2021. Poster 8016). Currently it is difficult to predict which patients will achieve deep responses from ide-cel. To identify clinical correlates of patients achieving CR/sCR with ide-cel, this subanalysis of KarMMa compared baseline characteristics between patients with CR/sCR and non-CR/sCR. Methods Patients with MM, ≥ 3 prior lines of therapy (including an immunomodulatory agent, PI, and anti-CD38 antibody), and disease refractory to last regimen per IMWG criteria received ide-cel infusion (target dose range 150-450 x 10 6 CAR+ T cells) after lymphodepletion (fludarabine 30 mg/m 2/day + cyclophosphamide 300 mg/m 2/day for 3 days). Bridging therapy was optional (last dose ≥ 14 days prior to lymphodepletion). The primary endpoint was overall response rate; CR/sCR rate was a key secondary endpoint. Baseline characteristics were collected prior to lymphodepletion and for select biomarkers on day of infusion. In this subanalysis, univariate and multivariate logistic regression models were used to identify baseline characteristics that correlate with the likelihood of achieving CR/sCR. Results In total, 128 of 140 patients received ide-cel infusions at data cutoff (Dec 21, 2020). There were 42 patients with best overall response of CR/sCR and 86 with non-CR/sCR (very good partial response [VGPR], partial response [PR], or no response). Among those with CR/sCR, 32 (76%) were negative for minimal residual disease (MRD) at a sensitivity level of < 10 -5 nucleated cells and 19 of these patients maintained MRD negativity at the 12-month follow-up. Baseline demographics and disease characteristics were generally balanced between patients with CR/sCR and non-CR/sCR; notable exceptions included revised International Staging System (ISS) stage III disease, IgG chain type, CD138+ plasma cell percentage, and β-2-microglobulin levels (Table 1). Univariate analysis of CR/sCR by baseline characteristics showed that IgG heavy chain versus other heavy chain types (odds ratio [OR]: 0.162, P < 0.0001), high sBCMA (OR: 0.646, P = 0.0007), β-2-microglobulin (≥ 5.5 vs < 3.5 mg/L; OR: 0.201, P = 0.0072), and presence of extramedullary disease (OR: 0.428, P = 0.0394) were negatively associated with CR/sCR, whereas high vector copy number in drug product was positively associated with CR/sCR (OR: 1.290, P = 0.0287). A multivariate analysis of CR/sCR identified IgG heavy chain versus other heavy chain types (OR: 0.100, P < 0.0001), high sBCMA (OR: 0.637, P = 0.0110), and elevated prothrombin time-international normalized test (OR: 0.005, P = 0.0365) as negative correlates of CR/sCR, and high vector copy number in drug product (OR: 1.486, P = 0.0168) as a positive correlate of CR/sCR. Descriptive analysis demonstrated lower baseline sBCMA in patients with CR/sCR versus non-CR/sCR; across both groups, sBCMA levels increased between screening and baseline (Table 2). Conclusions In this subanalysis of KarMMa, multivariate analysis identified IgG, sBCMA, and prothrombin time-international normalized test as negative correlates of CR/sCR, and vector copy number in drug product a positive correlate. As sBCMA is an indicator of tumor burden and can affect therapies targeting BCMA (Cowan et al. ASH 2019. Abstract 204), selecting for patients with lower tumor burden and controlling tumor burden during manufacturing or bridging therapy may be important in achieving CR/sCR with ide-cel. The impact of modulating tumor BCMA and sBCMA, using a gamma-secretase inhibitor prior to ide-cel infusion, is currently under evaluation in the KarMMa-7 trial. Study support bluebird bio and Celgene, a Bristol-Myers Squibb Company. Figure 1 Figure 1. Disclosures Shah: Indapta Therapeutics: Consultancy; Janssen: Research Funding; Sutro Biopharma: Research Funding; Poseida: Research Funding; Precision Biosciences: Research Funding; Amgen: Consultancy; Bluebird Bio: Research Funding; Teneobio: Research Funding; CareDx: Consultancy; GSK: Consultancy; BMS/Celgene: Research Funding; CSL Behring: Consultancy; Karyopharm: Consultancy; Sanofi: Consultancy; Kite: Consultancy; Nektar: Research Funding; Oncopeptides: Consultancy. Munshi: Abbvie: Consultancy; Pfizer: Consultancy; Janssen: Consultancy; Bristol-Myers Squibb: Consultancy; Amgen: Consultancy; Takeda: Consultancy; Oncopep: Consultancy, Current equity holder in publicly-traded company, Other: scientific founder, Patents & Royalties; Celgene: Consultancy; Karyopharm: Consultancy; Adaptive Biotechnology: Consultancy; Novartis: Consultancy; Legend: Consultancy. Berdeja: Lilly, Novartis: Research Funding; Poseida, Sanofi, Teva: Research Funding; GSK, Ichnos Sciences, Incyte: Research Funding; EMD Sorono, Genentech: Research Funding; Celularity, CRISPR Therapeutics: Research Funding; Bluebird bio, BMS, Celgene, CRISPR Therapeutics, Janssen, Kite Pharma, Legend Biotech, SecuraBio, Takeda: Consultancy; Abbvie, Acetylon, Amgen: Research Funding. Jagannath: Legend Biotech: Consultancy; Karyopharm Therapeutics: Consultancy; Janssen Pharmaceuticals: Consultancy; Bristol Myers Squibb: Consultancy; Sanofi: Consultancy; Takeda: Consultancy. Finney: bluebird bio: Current Employment, Current equity holder in publicly-traded company. Martin: Bristol Myers Squibb: Current Employment, Current equity holder in publicly-traded company. Agarwal: Bristol Myers Squibb: Current Employment, Current holder of individual stocks in a privately-held company. Rowe: Bristol Myers Squibb: Current Employment. Campbell: Bristol Myers Squibb: Current Employment, Current holder of individual stocks in a privately-held company. San-Miguel: AbbVie, Amgen, Bristol-Myers Squibb, Celgene, GlaxoSmithKline, Janssen, Karyopharm, Merck Sharpe & Dohme, Novartis, Regeneron, Roche, Sanofi, SecuraBio, Takeda: Consultancy, Other: Advisory board.

Detection of Vector Copy Number in Bicistronic CD19xCD22 CAR T Cell Products with Digital PCR

Chimeric antigen receptor (CAR) T cell therapy is a rapidly evolving immunotherapeutic treatment modality for adult and pediatric patients with a variety of cancers, which has been most extensively investigated in B-cell malignancies. Given that CAR T cell immunotherapy involves changing the genetic composition of a patient's T cells, this living drug presents unique safety and quality control challenges. Vector copy number (VCN), a measurement of transgene copies within a CAR T cell product, is a product-specific characteristic that must be quantified prior to patient administration as high VCN increases the risk of insertional mutagenesis. Historically, VCN assessment in CAR T cell products has been performed via qPCR. qPCR is reliable along a broad range of concentrations but has inherent limitations in its lower limit of detection and limit of quantification. Digital PCR (dPCR) methods were developed for absolute quantification of target sequences by counting nucleic acid molecules encapsulated in discrete, volumetrically defined partitions. Advantages of dPCR compared to qPCR include simplicity, reproducibility, lower limit of detection, and definitive quantification. In this present study, we developed an assay for analysis of the novel bicistronic UCD19x22 CAR T cell construct, which was developed in the laboratory of Dr. Terry Fry at the University of Colorado and will be moving in to clinical trials later this year. Custom primer-probe assays were designed using Primer Express v3.0.1 and the ThermoFisher Custom TaqMan Assay Design Tool. As an internal control, forward and reverse primers as well as a VIC-labeled probe specific to human albumin (NCBI gene 213, HGNC:399) were designed. Primers and a FAM-labeled probe assay, specific for the bicistronic CD19x22 CAR T cell product, were designed at the junction site between the two distinct CARs. This study compares two different digital PCR modalities: (1) droplet digital PCR (ddPCR) via the BioRad QX200 system which utilizes water-in-oil droplet partitions and (2) the QIAcuity digital PCR system utilizing a nanoplate-based partitioning platform. While dPCR is a newer methodology compared to ddPCR, the two apply parallel procedures, data generation, and analyses. The primer/probe assay was validated with qPCR, dPCR and ddPCR using patient samples from preclinical CAR T cell manufacturing production runs, as well as Jurkat cell subclones which stably express this bicistronic CAR T product. We successfully developed an assay to specifically detect and quantify our bicistronic CD19xCD22 CAR transgene. ddPCR confirmed the specificity of this assay to detect only the bicistronic CAR product without any signal detected in samples containing untransduced T cells or T cells transduced with CD19 only CARs. Additionally, our assay gives accurate, precise, and reproducible CAR T cell VCN measurements across qPCR, dPCR, and ddPCR modalities. We demonstrate that digital PCR strategies can be utilized for absolute quantification of CAR transgenes and VCN measurements, and that specific assays can be developed for detection of unique constructs. Future studies will evaluate the utility of this assay with digital PCR modalities in measuring CAR T cell persistence in clinical trial patient samples after receiving this novel CAR T cell product. Figure 1 Figure 1. Disclosures Fry: Sana Biotechnology: Current Employment, Current equity holder in publicly-traded company.

Droplet digital PCR allows vector copy number assessment and monitoring of experimental CAR T cells in murine xenograft models or approved CD19 CAR T cell-treated patients

Background Genetically engineered chimeric antigen receptor (CAR) T lymphocytes are promising therapeutic tools for cancer. Four CAR T cell drugs, including tisagenlecleucel (tisa-cel) and axicabtagene-ciloleucel (axi-cel), all targeting CD19, are currently approved for treating B cell malignancies. Flow cytometry (FC) remains the standard for monitoring CAR T cells using a recombinant biotinylated target protein. Nevertheless, there is a need for additional tools, and the challenge is to develop an easy, relevant, highly sensitive, reproducible, and inexpensive detection method. Molecular tools can meet this need to specifically monitor long-term persistent CAR T cells. Methods Based on 2 experimental CAR T cell constructs, IL-1RAP and CS1, we designed 2 quantitative digital droplet (ddPCR) PCR assays. By targeting the 4.1BB/CD3z (28BBz) or 28/CD3z (28z) junction area, we demonstrated that PCR assays can be applied to approved CD19 CAR T drugs. Both 28z and 28BBz ddPCR assays allow determination of the average vector copy number (VCN) per cell. We confirmed that the VCN is dependent on the multiplicity of infection and verified that the VCN of our experimental or GMP-like IL-1RAP CAR T cells met the requirement (< 5 VCN/cell) for delivery to the clinical department, similar to approved axi-cel or tisa-cel drugs. Results 28BBz and 28z ddPCR assays applied to 2 tumoral (acute myeloid leukemia (AML) or multiple myeloma (MM) xenograft humanized NSG mouse models allowed us to quantify the early expansion (up to day 30) of CAR T cells after injection. Interestingly, following initial expansion, when circulating CAR T cells were challenged with the tumor, we noted a second expansion phase. Investigation of the bone marrow, spleen and lung showed that CAR T cells disseminated more within these tissues in mice previously injected with leukemic cell lines. Finally, circulating CAR T cell ddPCR monitoring of R/R acute lymphoid leukemia or diffuse large B cell lymphoma (n = 10 for tisa-cel and n = 7 for axi-cel) patients treated with both approved CAR T cells allowed detection of early expansion, which was highly correlated with FC, as well as long-term persistence (up to 450 days), while FC failed to detect these events. Conclusion Overall, we designed and validated 2 ddPCR assays allowing routine or preclinical monitoring of early- and long-term circulating approved or experimental CAR T cells, including our own IL-1RAP CAR T cells, which will be evaluated in an upcoming phase I clinical trial.

Application of Droplet Digital PCR for the Detection of Vector Copy Number in Clinical CAR/TCR T-Cell Products

Background : Genetically engineered T cells have become an important therapy for B-cell malignancies. Measuring the efficiency of vector integration into the T cell genome is important for assessing the potency and safety of these cancer immunotherapies. Methods: A digital droplet polymerase chain reaction (ddPCR) assay was developed and evaluated for assessing the average number of lenti- and retroviral vectors integrated into Chimeric Antigen Receptor (CAR) and T-Cell Receptor (TCR)-engineered T cells. Results: The ddPCR assay consistently measured the concentration of an empty vector in solution and the average number of CAR and TCR vectors integrated into T cell populations. There was a linear relationship between the average vector copy number per cell measured by ddPCR and the proportion of cells transduced as measured by flow cytometry. Similar vector copy number measurements were obtained by different staff using the ddPCR assay, highlighting the assays reproducibility among technicians. Analysis of fresh and cryopreserved CAR-T and TCR engineered T cells yielded similar results. Conclusions: ddPCR is a robust tool for accurate quantitation of average vector copy number in CAR and TCR engineered T-cells. The assay is also applicable to other types of genetically engineered cells including Natural Killer cells and hematopoietic stem cells.

Application of Droplet Digital PCR for the Detection of Vector Copy Number in Clinical CAR/TCR T-Cell Products

Background : Genetically engineered T cells have become an important therapy for B-cell malignancies. Measuring the efficiency of vector integration into the T cell genome is important for assessing the potency and safety of these cancer immunotherapies. Methods: A digital droplet polymerase chain reaction (ddPCR) assay was developed and evaluated for assessing the average number of lenti- and retroviral vectors integrated into Chimeric Antigen Receptor (CAR) and T-Cell Receptor (TCR)-engineered T cells. Results: The ddPCR assay consistently measured the concentration of an empty vector in solution and the average number of CAR and TCR vectors integrated into T cell populations. There was a linear relationship between the average vector copy number per cell measured by ddPCR and the proportion of cells transduced as measured by flow cytometry. Similar vector copy number measurements were obtained by different staff using the ddPCR assay, highlighting the assays reproducibility among technicians. Analysis of fresh and cryopreserved CAR-T and TCR engineered T cells yielded similar results. Conclusions: ddPCR is a robust tool for accurate quantitation of average vector copy number in CAR and TCR engineered T-cells. The assay is also applicable to other types of genetically engineered cells including Natural Killer cells and hematopoietic stem cells.

Ligand-inducible, prostate stem cell antigen (PSCA)-directed GoCAR-T cells in advanced solid tumors: Preliminary results with cyclophosphamide (Cy) ± fludarabine (Flu) lymphodepletion (LD).

2536 Background: Cell-surface protein PSCA is upregulated in many solid tumors and correlates with disease stage. BPX-601, an autologous T-cell product expressing a PSCA-CD3ζ CAR and a rimiducid (Rim)-inducible MyD88/CD40 co-activation switch to augment T-cell proliferation and persistence, is designed to have enhanced efficacy in solid tumors vs traditional CARs. This ongoing first-in-human study assesses safety, biologic, and clinical activity of BPX-601+Rim in PSCA+ cancers. Updated results, including those from patients (pts) who underwent LD with Flu/Cy, are presented. Methods: BP-012 is a 2-part, open-label trial. Part 1 is a 3+3 dose escalation of BPX-601 (1.25–5.0x106 cells/kg; Day [D] 0) given prior to a single, fixed Rim dose (0.4 mg/kg; D7) in pts with previously treated PSCA+ metastatic pancreatic, gastric, or prostate cancers with measurable disease. Results: As of Jan-22-2019, 15 pts have received BPX-601±Rim. Two pts at the highest cell dose received Flu/Cy for LD on D−5 to D−3 before BPX-601; LD after Flu/Cy was 96.6% and 84.3%. Thirteen pts received Cy alone on D−3; in these pts, LD ranged from 0–68.6%. Rapid cell expansion by D4 was observed in all pts with peak vector copy number 8.3-fold higher with Flu/Cy (n = 2) vs Cy LD (n = 13). Serum IP-10, IL-6 and TNFα increased > 2-fold from baseline in ≥1 pt in all Rim cohorts, with 3- to 20-fold Rim-dependent cell expansion in 6 pts. No CRS or DLTs were reported. After Rim, one Flu/Cy pt experienced a serious Grade 2 AE (encephalopathy) related to BPX-601+Rim that resolved with IV steroids; despite time-matched nonserious Grade 1 pyrexia, the pt had no other CRS symptoms. After BPX-601+Rim and ≥1 scan, best responses were 8 SD and 3 PD (1 non-evaluable). With a median follow-up of 9.8 wks, time to next treatment (tx) after BPX-601 ranged from 2.7–22.1 wks (n = 8) and ongoing tx-free intervals range from 9.1–30.1 wks (n = 4). Conclusions: BPX-601+Rim was well-tolerated with manageable safety and early evidence of enhanced CAR T-cell expansion and prolonged persistence after Flu/Cy vs Cy. Additional pts will undergo Flu/Cy LD prior to BPX-601 with single- and repeat-dose Rim. Clinical trial information: NCT02744287.

Improved Lentiviral Vectors for the Cure of Hemoglobinopathies

Given that both Sickle Cell Disease (SCD) and beta-thalassemia (BT) are caused by mutations in the beta-globin gene, several lentivirus-based gene addition therapies have been developed. Results from recent trials indicate that the vectors used are safe; however, their efficacy inversely correlates with the severity of patients' hemoglobinopathy. The severity of the mutations (non-beta0 vs beta0) largely influences the outcome of the gene transfer. In fact, the data indicate that a relatively low number of integrations (in the range of 1-2 copies per genome) or vector copy number (VCN) is sufficient to cure patients whose mutations are categorized as non-beta0 and express relative high levels of endogenous hemoglobins (adult hemoglobin, HbA, and/or fetal hemoglobin, HbF). In contrast, the same level of VCN alleviates the transfusion regimen of patients with beta0 mutations, but it does not cure them. In addition, the lentiviruses currently used in clinical trials were engineered by different groups and to date no one has directly compared them side by side. In light of these limitations, here we describe a study that supplies a platform for rapid screening of lentiviral vectors expressing curative hemoglobin, based on the correlation between VCN and the increase in HbA levels. We also compared newly generated lentiviral vectors to vectors currently used in clinical trials. Our ultimate goal is to generate a new vector that can increase the yield of beta globin expressed per VCN in patients' cells. Using CRISPR-Cas9 we modified the erythroid Hudep-2 cell line (Kurita et al, 2013) to generate a clonal cell line, named Hudep #M13, which, upon differentiation, produces a hemoglobin variant (HbMut) that can be discriminated from that produced by the lentiviruses (HbA). In parallel, we immortalized erythroid progenitor cells isolated from a SCD donor (SCD #13), using the HPV16-E6/E7 expression system, which was introduced into the cells by lentiviral transduction. Using Hudep #M13, we compared the correlation between gene transfer and the production of HbA for 5 novel lentiviral vectors, indicated as ALS16-20. Our new vectors include the Ankyrin insulator in the 3' LTR (Breda et al 2012), the full beta-globin gene (including the native introns), the full 3' enhancer region, a combination of different portions of the beta-globin promoter, as well as modifications and inclusion of novel genomic elements from the locus control region (LCR). Our ALS- constructs were then compared to lentiviral vectors currently utilized in clinical trials. These constructs were reproduced based on information available from the literature (Negre et al, 2015; Miccio et al, 2008; and Boulad et al, 2014) and indicated as CV-1, CV-2, and CV-3, respectively. All these vectors contain the beta-globin gene with deletions in intron 2, different portions of the beta-globin promoter and/or 3' enhancer region, and different elements and sizes of the hypersensitive sites (HS) of the LCR. In Hudep #M13, linear regression analysis of the ratio of HbA to vector copy number (VCN) for each treatment, indicates that ALS17 and ALS20 yield roughly 40, 157 and 84% more HbA per copy than CV-1, CV-2 and CV-3, respectively. Similar increment in HbA% were confirmed on primary and immortalized (SCD #13) SCD erythroblasts derived CD34+ cells isolated from patients' blood. In these specimens, ALS20 maintained a 40% HbA increase compared to CV-1, when exploring a range of VCN from 0 to 3 with a linear mixed effects model. To assess the ability of these constructs to increase hemoglobin content in vivo, we are performing murine bone marrow transplants using thalassemic hematopoietic stem cells treated with CV1 and our two most powerful vectors. Based on most recently reported data (Thompson et al, 2018), 1 copy of the vector we reproduced as CV-1, makes on average 6.8g/dL of HbA. Hence, 1 copy of our best vector has the potential to make up to 9.5g/dL HbA. This could lead to a much greater clinical impact for patient with hemoglobinopathies, especially those who require higher Hb production to become transfusion independent, like patients with the beta0 genotype. The completion of these studies will provide not only a comparative analysis of our new best vector to those already in clinical trial, but also a way to predict how much therapeutic hemoglobin per vector copy number will be produced in the clinical setting. Disclosures Casu: Aevi Genomic Medicine, Inc: Research Funding; Ionis Pharmaceuticals, Inc.: Research Funding. Kwiatkowski:bluebird bio: Consultancy, Honoraria, Research Funding; Agios Pharmaceuticals: Consultancy, Research Funding; Novartis: Research Funding; Apopharma: Research Funding; Terumo: Research Funding. Rivella:Disc Medicine: Consultancy; Protagonist: Consultancy; Ionis: Consultancy; Meira GTX: Membership on an entity's Board of Directors or advisory committees.

Gene Therapy for Sickle Cell Anemia Using a Modified Gamma Globin Lentivirus Vector and Reduced Intensity Conditioning Transplant Shows Promising Correction of the Disease Phenotype

Background: Genetic transfer of an anti-sickling β87-globin lentiviral vector (LV) into hematopoietic stem cells (HSC) followed by myeloablative transplant has cured one child with sickle cell anemia (SCA) (NEJM 2017), although it was not successful in 7 subsequent adult SCA patients, and modifications to intensify ablative conditioning, improve HSC dose, gene transfer are underway (Blood 130 Suppl 1: 527, 2017). Based upon our preclinical data (Blood 2009), we embarked upon a Reduced Intensity Conditioning (RIC) Phase I/II Pilot Study on Gene Transfer in Patients with SCA with a modified γ-Globin LV (NCT02186418), hypothesizing this approach will be safe, feasible and efficacious; Moreover, RIC will have significantly less toxicity, costs, and be implementable in many transplant centers, including those in some of the resource-poor countries, where supportive therapies for myeloablative transplants are unavailable, and where majority of SCA patients exist. Methods: Adult patients with severe SCA deemed eligible were transfused/erythrocytapheresed prior to HSC collection and transfused for 6 months post-transplant (PT) to Hb>10g/dl and HbS~30%. CD34+ HSC were collected via bone marrow harvest (BMH) and/or plerixafor mobilized Peripheral Blood Stem Collection (PBSC), selected for CD34+ cells and transduced. Patients received a single dose of IV melphalan (140mg/m2 BSA) 36hr prior to infusion of γ-globin modified (GM)-HSC. Patients were monitored for adverse events (AE), engraftment, vector copy number (VCN), modified HbF (HbF*) expression and clinical features of SCA. Results: Two SCA patients (35yo and 25yo) with HbS-β0 thalassemia genotype were treated. CD34+ HSC were collected via multiple BMH (P1) and BMH+PBSC (P2). Follow up data are available for 6 and 12mo on P1 and P2. P1 received 1x106 CD34+ cells/kgbw [vector copy number (VCN) 0.22], and P2 received 6.9x106 CD34+ cells/kgbw [VCN 0.46]. Time to neutrophil engraftment (ANC ≥ 500) was day 9 and 7 post-transplant (PT) in P1 and P2, respectively, and time to Plt recovery (Plt>50K) was day 14 PT in both. Patients included in this trial had severe disease and continued to have pre-existing chronic pain requiring significant opiates; hence ~80% of the AEs were pain events; other AEs were anticipated transient laboratory AEs associated with melphalan. Following GM-HSC infusion, both patients showed a progressive rise in HbF* (a point mutation in the γ-globin LV allows distinction from endogenous HbF by HPLC) starting from day 30 PT. Since patients had transfused HbA containing RBCs in the initial 6 months, HbF*/(HbF*+HbS) was calculated, and was 20% and 21% in P1 and P2 at day 180 PT and VCN 0.2-0.4, detected in all lineages. Integration site analysis, performed on the infused products (Day 0), at day 30 PT on P1 and P2, and on day 180 PT on P1 demonstrated highly polyclonal pattern of integration. At 1 yr PT, P1 had 20% HbF* (2.1g/dl HbF*, total Hb 10.6) with a stable VCN of 0.2-0.4 in multiple lineages in bone marrow and peripheral blood. The baseline Hb of P1 was 7.5-8.5g/dL prior to transplant. In the preceding 2 years prior to transplant, both patients were admitted for pain crises/acute chest >5-6 times/yr, and had chronic pain requiring chronic opiates. Chronic pain persisted for 4-5 months PT in P1, after which P1 has not required IV opiates, negligible oral opiates and has had no hospital visits/admissions with acute sickle events. P2 has required decreasing amounts of oral opiates for chronic back pain. Conclusions: Early results from 2 SCA adults treated with a modified γ-globin LV modified autologous HSC following RIC transplant showed excellent safety, feasibility, with minimal post-transplant toxicity, rapid count recovery, and sustained stable genetically modified cells in peripheral blood and bone marrow. The first patient shows significant clinical amelioration of the SCA phenotype at 1 year PT, with 20% vector-derived HbF (HbF*) that has caused amelioration of anemia, near elimination of chronic pain and absence of acute sickle events. The second patient, although still early post-transplant shows a similar HbF* trajectory. Additional study data will demonstrate whether this level of HbF* will provide consistent clinical benefit to patients with severe SCA. These early results, especially following a RIC transpant, are extremely promising; and if sustained, will provide a 'transportable' safe and feasible gene therapy for SCA. Disclosures Malik: CSL Behring: Patents & Royalties. Quinn:Global Blood Therapeutics: Research Funding; Silver Lake Research Corporation: Research Funding; Amgen: Research Funding.