Bacteria have been investigated for cancer for many centuries, as exemplified by William Coley’s injections of S. pyogenes and S. marcescens into patients with inoperable bone and soft tissue sarcomas that induced remissions1,2. Since then, attenuated strains of bacteria such as M. bovis, Bifidobacterium spp., S. typhimurium, L. monocytogenes, C. Novyi, have been engineered to deliver therapeutics for solid tumors of bladder, liver, colon, breast and brain in clinical trials (NCT02015104, NCT03358511, NCT04589234, NCT01967758, NCT01924689). There has been little attention paid to primary lung cancers yet there are several reports of an existing lung tumor microbiome3–6 presenting a natural case for bacterial therapies. Lung cancer is responsible for the highest number of cancer-related deaths worldwide and is also one of the most heterogenous type of cancer7–9. This heterogeneity of lung cancer along with the large search space of parameters for engineering bacterial strains, genetic circuits, and therapeutic payloads has produced a challenge in narrowing down a specific bacterial therapeutic for a particular type of lung cancer. Here, we focus on non-small cell lung cancer (NSCLC) the most common of lung cancer cases10. We establish an experimental pipeline to identify and characterize (Fig. 1a) therapeutics delivered by Salmonella typhimurium to match with NSCLC models and combine their benefit with current targeted therapies.
To identify bacterial toxins as effective therapeutics in lung cancer, we initially tested the viability of a subset of genetically diverse NSCLC cell lines upon exposure to an array of bacterial toxins. We chose a diverse array of NSCLC lines (ATCC): H460 has overexpression of TP53 mRNA, A549 has mutated KRAS, and HCC827 has mutated EGFR. H1819 and H2009 were derived from patients after administration of chemotherapy and radiation. H460 was derived from pleural fluid while H1792 was derived from a metastatic site. The bacterial toxins were previously engineered to be produced under inducible circuits of S. typhimurium EHL130111,12. We induced therapeutic production in S. typhymurium and applied bacterial lysates to cells, which identified θ toxin with the highest effect on viability among all bacterial toxins tested (Fig. 1b, Suppl. Figure 1). θ toxin is a pore forming toxin from C. perfringens13 and was previously shown to have efficacy against murine colon cancer cells11. To model more physiologic conditions that occur within tumors containing poorly vascularized hypoxic regions where therapeutic efficacy is thought to be reduced14, we tested the effect of θ toxin on spheroids derived from a subset of our cell lines. We found that lysate of S. typhymurium expressing θ toxin (Stθ) induced significantly higher cell death (p < 0.001) within the spheroids in a dose-dependent manner, as compared to lysate of non-induced S. typhymurium or lysate of S. typhymurium alone (Fig. 1c). Furthermore, these spheroids responded to purified θ toxin as well (ATCC, BTX-100) above 0.5 µg/mL concentration (Suppl. Figure 2). Taken together, we identified θ toxin as a promising therapeutic to be delivered by bacteria for several NSCLCs.
To selectively deliver therapeutics to tumor cells, bacteria need to be able to efficiently colonize tumors in spite of the production of payloads. Previous studies have established that live S. typhymurium can colonize murine colon cancer spheroids due to tumor-specific signatures, such as hypoxia, acidic pH and lactate15–17. We first assayed if live S. typhymurium were capable of colonizing hypoxic regions within the NSCLC spheroids. We found that the GFP-labeled bacteria colonized near dye-labeled hypoxic regions within the spheroid core (Fig. 1d), where they persisted for the duration of one week (Suppl. Figure 3). Next, we assayed the viability of NSCLC spheroids containing live S. typhymurium when θ toxin expression was induced with AHL. We found that live Stθ significantly reduced viability of the majority of NSCLC spheroids, as compared to colonized live S. typhymurium alone (Fig. 1e). An extended analysis demonstrated that reduced viability was achieved in all spheroids within 2 weeks of Stθ exposure (Suppl. Figure 4). The ability of live Stθ to persistently colonize the spheroid core, while robustly reducing viability of heterogenous NSCLC cell types, encouraged the extension of this approach to in vivo conditions.
We next explored the efficacy of Stθ in a mouse tumor model, which would locally deliver θ toxin in vivo while reducing systemic toxic effects. Specifically, we administered live bacteria intratumorally in tumor xenografts grown from a NSCLC cell line (H460) and induced the production of θ toxin 1 day after bacteria injection (Fig. 2a). This assay resulted in a three-fold reduction in tumor growth within a week (Fig. 2b, top, p-value < 0.001, Suppl. Figure 5). Importantly, tumor control was achieved without inducing systemic toxicity after administration of high concentration of Stθ (4.5 X 108 CFU/mL and 40 µL per tumor), as assessed by weight change (Fig. 2b, bottom), and the absence of detectable S. typhymurium via IHC assay from peripheral organs at the endpoint (Fig. 2c, top). In addition, none of the peripheral organs showed signs of apoptosis, as indicated by the lack of cleaved caspase 3 signal, compared to the tumors (Fig. 2c, bottom, Suppl. Figure 6). As immortalized Human Bronchial Epithelial Cells (HBECs) showed moderate level of response to the toxins (Suppl. Figure 7), we routinely examined behavioral points of the mice such as activity, aggression, bite reflex, posture, presence of straub tail, seizures, and overall signs of morbidity upon Stθ exposure. Our experiments demonstrated lack of these behavioral changes and point to the safety of this approach, reducing concerns regarding potential systemic toxicity associated with intratumoral use of live S. typhymurium in vivo.
Can we improve Stθ by combining with current standard of care chemotherapies as well as small molecule inhibitors being tested in clinical trials? To narrow down potential drugs to combine with Stθ treatment, we aimed to gain mechanistic insight into cellular pathways altered in two of the Stθ-responder spheroids (H460 and H1819, Fig. 3a). We compared genome-wide transcriptional profiling (RNA-seq) of tumor cells derived from spheroids housing live S. typhymurium in their hypoxic cores in presence or absence of producing θ toxin. Gene Set Enrichment Analysis (GSEA) of Next-Gen sequencing revealed significant changes in at least 10 signaling pathways shared by both NSCLC cell lines upon Stθ treatment (Fig. 3b and Suppl. Figure 8, normalized p-value < 0.5 and falls discovery rate q-value < 0.5). Specifically, cell cycle checkpoint, PI3K/AKT/mTOR signaling and DNA repair pathways were among the most significantly enriched gene sets in both H460 and H1819. As top 50 differentially expressed genes from H1819 and H460 did not show modest overlap (Suppl. Figure 9), we selected 7 small molecule inhibitors specific to lung cancer therapeutic landscape that are well known (NCT01294306, NCT00744900, NCT03392246)18,19 to target the significantly enriched pathways in our analysis. We hypothesized - if under Stθ treatment, H1819 and H460 are dependent on the signaling through these pathways for their survival, a combinatorial strategy with each drug and Stθ would eliminate more cancer cells than the single treatments. Indeed, when combinatorically treated with each drug and Stθ, 4 out of 7 combinations showed robust efficacy across both spheroids (Fig. 3c) compared to drug-only or lysate of Salmonella expressing θ toxin. Notably, H1819 showed more resistance to 3 out of 7 combination therapy, and may represent more drug-resistant disease, as it was derived from a patient previously treated with both chemotherapy and radiation (ATCC). Interestingly, this benefit was not observed when the combinatorial treatment was tested on spheroids derived from mouse lung cancer cells with genetically modified TP53 and KRAS (Suppl. Figure 10) suggesting this improved efficacy is specific to H1819 and H460 as indicated by their transcriptional profiling. Taking AKT signaling as an example we observed high expression of phopho-AKT (Ser473) upon treatment with bacterial θ toxin and this signal was reduced in presence of MK2206 drug (Fig. 4a, b) that is known to block AKT-phosphorylation at Ser473. This data suggests biological changes induced by the θ toxin treatment in NSCLC cells can be exploited to design new combination therapies. Specifically, NSCLC cells may upregulate AKT phosphorylation upon θ toxin treatment and blocking AKT phosphorylation with MK2206 achieves improved efficacy.
Lastly, to test the efficacy of this combination therapy in vivo, we treated tumor xenografts with Stθ and MK2206 drug. For effective comparison with MK2206 + Stθ treatment, we designed multiple control cohorts: vehicle only, MK2206 alone, Stθ alone, live S. typhymurium alone, and, MK2206 + live S. typhymurium. Additionally, to observe the benefit of combination therapy at lower dosage and earlier time points, we intratumorally dosed bacteria (4.5 X 107 CFU/mL and 20 µL per tumor) and started treatment when tumors reached ~ 150 mm3. The cohort treated with MK2206 alone or with live bacteria producing no toxin, showed no difference in tumor growth from the cohort treated with only vehicle (Fig. 4c). However, mice that received MK2206 + Stθ treatment showed significantly reduced tumor growth (p-value < 0.001) compared to other control cohorts. No significant reduction in total body weight was detected in any treatment groups (Suppl. Figure 11). Taken together, Stθ showed improved efficacy and no additional toxicity when combined with MK2206 targeted therapy in vivo.