Combination Cancer Therapy Using Chimeric Antigen Receptor-Engineered Natural Killer Cells as Drug Carriers

Abstract

The therapeutic limitations of conventional chemotherapeutic drugs include chemo-resistance, tumor recurrence, and metastasis. Numerous nanoparticle-based active targeting approaches have emerged to enhance the intracellular concentration of drugs in tumor cells; however, efficient delivery of these systems to the tumor site while sparing healthy tissue remains elusive. Recently, much attention has been given to human immune-cell-directed nanoparticle drug delivery, because immune cells can traffic to the tumor and inflammatory sites. Natural killer cells are a subset of cytotoxic lymphocytes that play critical roles in cancer immunosurveillance. Engineering of the human natural killer cell line, NK92, to express chimeric antigen receptors to redirect their antitumor specificity has shown significant promise. We demonstrate that the efficacy of chemotherapy can be enhanced in vitro and in vivo while reducing off-target toxicity by using chimeric antigen receptor-engineered NK92 cells as carriers to direct drug-loaded nanoparticles to the target site.

Keywords: cancer immunotherapy; chimeric antigen receptors; drug delivery; nanomedicine.

Figure 1

NK92 Cell Conjugation to Maleimide-Functionalized cMLVs (A) Schematic of CAR.NK cells conjugated to PTX-loaded cMLVs. CARs are derived from the single-chain variable fragment (scFv) of an antibody and the T cell receptor signaling complex. CARs can be transduced into NK92 cells, and cMLVs can conjugate to the cell surface by interacting with free thiols. (B) cMLVs conjugated to the NK cell surface at various cMLV:cell ratios. cMLVs containing the fluorescent dye DiD were coincubated with NK cells over a range of ratios. The number of cMLVs on the surface of each cell was calculated by analyzing the DiD fluorescence. The ratio of 1,000:1 provided the maximum amount of cMLVs per cell and was used in future experiments. (C) Confocal microscopy of CAR.NK cells conjugated to DiD-loaded cMLVs [cMLV(DiD)]. CAR.NK cells were labeled with 1 μM CFSE and washed with PBS prior to conjugation to cMLV(DiD). Confocal microscopy was used to visualize the cMLVs on the CAR.NK cell surface. Scale bars, 5 μm. (D) Internalization assay of conjugated cMLVs. CAR.NK cells were conjugated with carboxyfluorescein-tagged maleimide-labeled cMLVs. The extracellular conjugation was quenched by trypan blue to differentiate surface-bound and internalized cMLVs 2 hr after conjugation. Attachment of cMLVs to CAR.NK cells did not trigger the internalization of particles by the cells. Summarized statistics are displayed in the graphs (n = 3, mean ± SEM; ***p < 0.001). NS, not significant.

Figure 2

Cytotoxicity of CAR.NK Cells against CD19⁺ or Her2⁺ Target Cells (A) Cytotoxicity of anti-CD19 CAR.NK cells. Anti-CD19 CAR.NK cells were cocultured with CD19⁻ SKOV3 cells or CD19⁺ SKOV.CD19 cells for 24 hr at 1:1, 5:1, or 10:1 effector-to-target ratios, and cytotoxicity was measured. (B) Cytotoxicity of anti-Her2 CAR.NK cells. Anti-Her2 CAR.NK cells were cocultured with Her2⁻ MDA.MB.468 cells or Her2⁺ SKOV3 cells for 24 hr at 1:1, 5:1, or 10:1 effector-to-target ratios, and cytotoxicity was measured. Summarized statistics are displayed in the graphs (n = 3; mean ± SEM; *p < 0.05; **p < 0.01; ***p < 0.001). NS, not significant.

Figure 3

CAR.NK Cytokine Release and Migration When Conjugated to cMLVs (A and B) IFN-γ staining assays. Anti-CD19 (A) or anti-Her2 (B) CAR.NK cells were cocultured with various target cells with brefeldin A protein transport inhibitor for 6 hr to detect IFN-γ release. Unstimulated CAR.NK cells served as a negative control. CAR.NK cells were either unconjugated or conjugated with empty cMLVs [CAR.NK.cMLV(EMPTY)] or PTX-loaded cMLVs [CAR.NK.cMLV(PTX)]. IFN-γ was measured with intracellular staining. (C and D) Cytotoxicity assays. Anti-CD19 (C) or anti-Her2 (D) CAR.NK cells were cocultured with various target cells at a 1:1 ratio for 24 hr, and cytotoxicity was measured. CAR.NK cells were either unconjugated or conjugated with empty cMLVs [CAR.NK.cMLV(EMPTY)] or PTX-loaded cMLVs [CAR.NK.cMLV(PTX)]. (E) Migration assay. Unconjugated NK or NK conjugated to cMLV(EMPTY) or cMLV(PTX) were plated in the upper chambers of a Transwell plate. Negative controls had plain media in the lower wells, and CXCL9 was used as a chemoattractant in the lower wells of non-control groups. After 6 hr of incubation, media from the lower chambers were collected and NK cells were counted. Summarized statistics are displayed in the graphs (n = 3; mean ± SEM; *p < 0.05; **p < 0.01; ***p < 0.001). NS, not significant.

Figure 4

Biodistribution of Free cMLV(DiD) and Conjugated CAR.NK.cMLV(DiD) (A–F) Biodistribution data 24 (A and B), 48 (C and D), or 72 hr (E and F) after intravenous injections. NOD/SCID/IL2rγ^−/− (NSG) mice bearing subcutaneous SKOV3.CD19 tumors were intravenously injected with 2 × 10⁷ CAR.NK cells conjugated with DiD-labeled cMLVs or an equivalent number of DiD-labeled cMLVs alone (n = 3 per group per time point). After 24, 48, and 72 hr, indicated tissues were removed, weighed, and macerated with scissors. Specific DiD tissue fluorescence for each organ was quantified using the IVIS Spectrum imaging system, and the mean percentage of injected dose per gram of tissue (% ID/g) was calculated as the final readout. Summarized statistics are displayed (n = 3, mean ± SEM; *p < 0.05; **p < 0.01; ***p < 0.001) (A, C, and E), and images of the tumors from each of the three mice for cMLV(DiD) and CAR.NK.cMLV(DiD) are shown (B, D, and F). NS, not significant.

Figure 5

Antitumor Efficacy of CAR.NK.cMLV(PTX) in Solid Tumor Xenograft Model Tumor growth curve. SKOV.CD19 cells were injected subcutaneously into the right flank of NSG mice on day 0. Mice were randomized into six groups (n = 5 per group) and treated according to their group description four times total, 3–4 days apart via tail-vein injection. Tumor size was measured with a fine caliper (n = 5; mean ± SEM; *p < 0.05; **p < 0.01; ***p < 0.001). NS, not significant.

Figure 6

Ex Vivo Analysis of CAR.NK.cMLV(PTX) Treatment (A) Intratumoral PTX concentration. Thawed tumor samples were homogenized and PTX concentrations were analyzed using HPLC (n = 3; mean ± SEM). (B) TUNEL assay of fixed frozen tumor sections. Tumor sections were stained with a TUNEL kit according to the manufacturer’s instructions and imaged with confocal microscopy. Representative images are shown herein. Scale bars, 50 μm. (C) Quantification of TUNEL⁺ cells from fixed frozen tumor sections (n = 16; mean ± SEM). (D) Histology slides for cardiac toxicity. Cardiac tissue was fixed and frozen, and sections were mounted on glass slides. The frozen sections were stained with H&E. Histopathologic specimens were examined by light microscopy. Representative images are shown herein. **p < 0.01; ***p < 0.001; ****p < 0.0001. NS, not significant.