PLK1 inhibition upregulates PD-L1 expression
We and others have previously reported that PLK1 inhibition or knock-down results in cell cycle arrest in G2/M leading to cancer cell death.13-15 Herein, we found that PLK1 knockdown also results in an increase in PD-L1 surface expression in both human (A549) and murine (LLC-JSP) lung cancer cell lines. As shown in Fig. 1A, 85% knockdown of PLK1 mRNA (by siRNA against PLK1) resulted in 2.5-fold increase in PD-L1 mRNA expression in A549 cell line compared with scrambled siRNA (siSCR) treated cells. This was then confirmed at the surface protein level in A549 (Fig. 1B) and LLC-JSP (Fig. 1C) lung cancer cell lines at 3 days post siRNA treatments.
Mitotic kinase inhibitor (MKI)-induced PD-L1 expression
Following on the discovery that PLK1 inhibition results in PD-L1 upregulation, we sought to determine whether this holds true for inhibition of other mitotic kinases. We screened three leading mitotic kinase small molecule inhibitors against PLK1 (volasertib), Aurora kinase A (alisertib), and CHK1 (AZD7762) in human and mouse lung cancer cell lines. As shown in Supplementary Fig. 1, treatment of LLC-JSP (a murine lung cancer cell line) with volasertib, alisertib, or AZD7762 led to significant cell death (Supplementary Fig. 1A) and upregulated surface PD-L1 level (Supplementary Figs. 1B&C) in each case. This not only establishes the link between mitotic kinase inhibition and PD-L1 upregulation, but also demonstrates the potency of volasertib over other mitotic kinase inhibitors.
Combination of PLK1 inhibition and PD-L1 blockade reduces tumor growth and prolongs survival in mice
Based on our finding that PLK1 inhibition upregulated PD-L1 protein, we aimed to investigate whether the PLK1 inhibitor volasertib and a PD-L1 monoclonal antibody would synergize in vivo. We used LLC-JSP cell line to develop a flank tumor model in immune-competent mice similar to previous report.16 Mice with tumors (>60 mm3) were treated i.p. with PLK1 inhibitor volasertib and PD-L1 antibody as shown in Fig. 2A. The combination treatment (volasertib + PD-L1 antibody) significantly reduced tumor growth (Fig. 2B) and prolonged survival of mice (Fig. 2C) compared to each monotherapy.
PD-L1 antibody conjugated nanoparticles for delivery of PLK1 inhibitor volasertib (ARAC)
Our data show for the first time the benefit of combining PD-L1 antibody and PLK1 inhibitor to enhance therapeutic impact. However, each drug alone carries significant toxicity risks which may limit the clinical translation of this combination. To overcome this limitation, we investigated a targeted delivery approach utilizing our polymer modified mesoporous silica nanoparticle (NP) platform. The same nanoparticle platform has been proven effective for targeted delivery of siRNA to breast tumors and lung tumors in our prior work.12-14,17 Volasertib (iPLK1) was loaded onto mesoporous silica nanoparticle (MSNP) core prior to surface modification with polyethylenimine (PEI), polyethylene glycol (PEG), and PD-L1 antibody (Fig. 3A). The final composition contained 15% PEI and 13% PEG (by TGA), with 0.5% volasertib (by HPLC-UV-Vis), and 4.0% PD-L1 antibody (by BCA) (by weight of MSNP). Volasertib loaded nanoparticles, with PD-L1 antibody (p-iPLK1-NP; ARAC) and without (iPLK1-NP), had a size of 90 nm (Fig. 3B). As shown in Fig. 3C, treatment of LLC-JSP cells with iPLK1-NP significantly reduced cell viability more than the free volasertib counterpart given at the same dose. Similar results were obtained with B16-F10 melanoma cells and 4T1 breast cancer cells (Supplementary Fig. 2). In this case, PD-L1 antibody (see ARAC vs. iPLK1-NP; Fig. 3D) had no role in cancer killing (since there were no T cells in the system) or enhancing the delivery (since all nanoparticles were taken up by cells within 3 days regardless of having PD-L1 antibody or not). In agreement with previous finding using PLK1 siRNA (Fig. 1), treatment with iPLK1-NP resulted in significant increase in surface PD-L1 expression of the surviving cells at 2 days (about 60% cells were dead), but not at 2 hrs (Fig. 3E) since PLK1 inhibition effects (i.e. cell cycle arrest and death) had not yet transpired. However, PD-L1 antibody on the nanoparticles was as effective as free PD-L1 antibody, given at a 30-fold higher dose, at reducing surface PD-L1 level (see ARAC at 2 hrs, Fig. 3E). This is owing to the high local concentration of antibodies on the nanoparticles that the cells encountered. Nanoparticles with dense PD-L1 antibodies (approximately 2 x 103 antibodies per particle) bind multiple PD-L1 ligands on cell surface at once and are endocytosed with PD-L1 (termed receptor-meditated endocytosis),18 resulting in PD-L1 degradation (e.g., in lysosomes). Furthermore, Fig. 3E suggests that PD-L1 upregulation in the surviving population upon treatment with iPLK1-NP is not via selection process (e.g., death of cells with low PD-L1 first) since untreated cells (PBS) did not have a high PD-L1 population), but rather is due to signaling effects of PLK1 inhibition. Indeed, PLK1 inhibition also led to upregulation of PD-L1 expression in melanoma and breast cancer cells with varying PD-L1 baseline expression (Supplementary Fig. 2).
Feed-forward delivery and specificity of ARAC nanoconstruct
While ARAC initially reduces PD-L1 levels upon binding and internalization (as shown in Fig. 3E), surviving cells have upregulated PD-L1 due to the signaling effects of PLK1 inhibition. In this context, upregulated PD-L1 is used as the homing target for subsequent ARAC, leading to cancer targeting in a feedforward manner (i.e., higher targeting with increased doses of the treatment). To investigate the feedforward targeting of ARAC, we used 4T1 murine cancer cells which express low baseline PD-L1 levels. ARAC led to the upregulation of PD-L1 in 4T1 cells 4 days post treatment (Fig. 4A). We then assessed the cellular uptake of ARAC in control 4T1 cells (with low PD-L1) and ARAC-treated 4T1 cells (with upregulated PD-L1). As shown in Fig. 4B, after 1 hour of exposure, ARAC was preferentially taken up by the PD-L1 high cells vs. PD-L1 low cells by nearly 4-fold, demonstrating the selectivity and feed-forward targeting by ARAC. We also evaluated the cell killing selectivity by comparing viability of murine cancer cells (LLC-JSP, 4T1, B16-F10) vs. bone marrow-derived dendritic cells (BMDC) after treatment with ARAC. As shown in Fig. 4C, ARAC led to significant cell killing in cancer cells but minimal killing in dendritic cells. Similar to Fig. 3D, PD-L1 antibody has no effect on enhancing the delivery in this setting since all nanoparticles are taken up by cells within 3 days regardless of PD-L1 expression. Thus, the treatment selectivity to cancer cells over BMDC cells is due to cancer dependence on PLK1, as previously reported.18
ARAC induces systemic anti-tumor immune response in a bilateral lung cancer tumor model
To assess the anti-tumor immune response of ARAC, we utilized a bilateral flank tumor model. C57BL/6 mice were injected with 100K and 40K LLC-JSP cells on the right and left flank, respectively. At day 12 post injection, the right flank (local) tumors were injected with PBS, p-NP (NP with PD-L1 antibody), iPLK1-NP (NP with volasertib), or ARAC as shown in Fig. 5A. Growth of local (treated) and distant (untreated) tumors were monitored. Treatments with ARAC significantly reduced growth of local tumors compared with p-NP or iPLK1-NP (Fig. 5B). Importantly, a significant delay in the onset of distal tumors was also observed in ARAC group (Fig. 5C), suggesting that whole-body anti-tumor immune response was generated. Further, ARAC significantly prolonged survival of mice vs. saline or single drug NPs (Fig. 5D). In a separate study, mice were injected with saline or ARAC as shown in Fig. 5A and injected (local) tumors were harvested one day post last treatment to assess T cell infiltration. ARAC treated tumors had significantly higher infiltrating lymphocytes (CD3+ T cells), specifically cytotoxic CD8+ T cells, than the control (Fig. 5E).
NP delivery reduces effective doses of PLK1 inhibitor and PD-L1 antibody in lung tumor mice by 5-fold
To evaluate ARAC systemically, we developed an experimental metastatic lung tumor model by intravenous injection of LLC-JSP cells (200K cells), which developed tumors mainly in the lungs (confirmed at sacrifice). Mice were randomly grouped and treated intravenously (i.v.) via tail vein with saline, free drugs (volasertib + PD-L1 antibody at same dose or 5-fold higher dose than dose on ARAC), ARAC, or ARAC plus anti-CD8 antibody (Fig. 6A). Mice treated with ARAC survived significantly longer than those treated with saline or free drugs at same dose (***p<0.001 vs. saline; **p<0.01 vs. free drugs (1x)) (Fig. 6B) and slightly better than those treated with the 5-fold dose of the free drug combo (*p<0.05 vs. saline). Thus, delivery with ARAC could effectively reduce required dose of drug by at least 5-fold. Moreover, ARAC’s efficacy was confirmed to be immune-mediated as CD8+ T cell depletion by anti-CD8 antibodies abolished the prolonged survival of ARAC-treated mice (Fig. 6C). Furthermore, treatment with ARAC did not cause any weight loss, demonstrating its safety in mice (Fig. 6D).
Incorporating CpG adjuvant to ARAC (ARAC-CpG) enhances anti-tumor immune effects resulting in cures
We investigated whether the CpG adjuvant could further stimulate anti-tumor immunity and improve tumor control in a bilateral tumor model shown in Fig. 5. CpG 1826 (class B CpG for mice) was loaded on ARAC at 4% by weight of MSNP (complete loading confirmed by Nanodrop Spectrophotometer) with no significant change in hydrodynamic size (Fig. 7A). Both ARAC and ARAC-CpG significantly reduced tumor growth compared with saline-treated mice (Fig. 7B). Moreover, ARAC-CpG resulted in complete cure in 2 out of 7 mice (Fig. 7C), while no cures were achieved in ARAC-treated mice. CpG oligodeoxynucleotides act as a danger associated molecular pattern (DAMP) to stimulate pattern recognition receptors, specifically the toll-like receptor 9 (TLR9). This leads to the activation of antigen presenting cells and subsequent priming of T cells. Thus, by releasing antigens (via cancer killing by volasertib), delivering CpG adjuvant, and blocking PD-L1 immune checkpoints, ARAC-CpG tackles various strategies by which cancer cells evade the immune response; hence complete cures were achieved with this strategy.
Safety of nanoparticle platform in Cynomolgus monkeys
To date, we found our nanoparticle (NP) platform conjugated with trastuzumab (HER2 antibody) for delivering of HER2 siRNA to meet required safety criteria: (1) Low cytotoxicity of multiple organ cells (<10% cell death),17 (2) Great blood compatibility,12 (3) Not triggering adverse immune response of blood immune cells (PBMC),12 (4) Excellent safety after 9 doses given to mice over 1 month by not causing adverse effects to body weight, serum biomarkers, and histology of kidney and liver,19 (5) Good maximum tolerated dose up to 5-fold of efficacious dose,19 and (6) effective clearance as MSNP is soluble to benign silicic acid20,21 at serum pH and cleared in urine.19 Herein, we report preliminary toxicology study of nanoparticle platform co-delivering a PLK1 inhibitor (siRNA) and PD-L1 antibody (avelumab) in non-human primates (NHP), conducted by Charles River Lab (CRL). Avelumab (Merck/Pfizer) is an FDA approved PD-L1 antibody and was selected as the PD-L1 antibody as it has the highest binding affinity compared with other approved PD-L1 antibodies and also causes antibody-dependent cell cytotoxicity (ADCC) effects.22 Cynomolgus monkeys (n=3) received intravenous infusion of 5.6 mg/kg bare NP, 6 mg/kg ARAC (estimated efficacious dose) and 18 mg/kg ARAC (3-fold efficacious dose), with a one week washout period in between dosing. Clinical signs, body weights, food consumption, dermal observations, clinical pathology parameters (hematology, coagulation, clinical chemistry, and cytokine secretion), gross necropsy findings, organ weights, and histopathologic examinations were evaluated. There were no test article-related clinical observations or effects on body weight (Supplementary Table 1), food consumption, or coagulation (Supplementary Table 3). Dermal observations of erythema and edema were noted at 6 and 24 hours post dose but resolved at 48 hrs post dose (Supplementary Table 6). For hematology, nonadverse decreased white blood cell (WBC), neutrophil, and lymphocyte counts were observed in 2 of 3 monkeys at the highest dose (18 mg/kg ARAC) on day 2 post dose but were resolved by day 7 post dose, indicating recovery (Table 1; Supplementary Table 2). For clinical chemistry, nonadverse increased aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels were noted on day 2 following administration of NP alone, 6 mg/kg ARAC, and 18 mg/kg ARAC. However, for each group, AST and ALT levels of day 7 were similar to predose values, indicating recovery (Table 1; Supplementary Table 4). Further, key cytokines (IL-1b, IL-6, IFN-g, TNF-a, IFN-a, and MCP-1) were monitored in serum collected at 0, 6, 24, and 48 hours. Even in monkeys receiving the highest dose (18 mg/kg), only MCP-1 and IFN-a were mildly to moderately elevated in 2 out of 3 monkeys at 6 hrs, which subsided at 24 hrs without intervention (Supplementary Table 5). Increase of these cytokines right after treatment may be part of therapeutic actions (e.g., IFN-a suggests induction of innate immunity by siRNA;23,24 PD-L1 inhibition induced a higher production of MCP-1).25 Terminal euthanasia and necropsy was performed one week following the 18 mg/kg dosing to assess gross pathology, organ weights, and histopathology. A few observations were noted that were considered incidental and of the nature commonly observed in this species and age of monkeys. Thus, there were no test article-related effects on survival, organ weights, gross pathology, or histopathology (Supplementary Tables 7-9). In conclusion, the nanoconstruct was found to be safe and well tolerated in NHP at up to 3-fold anticipated efficacious dose. Based on these results, the no‑observed‑adverse-effect level (NOAEL) for ARAC was considered to be 18 mg/kg. Dose-limiting toxicity (DLT) of ARAC was not reached in this preliminary study and will be determined in subsequent NHP studies.