SM-88 Shows Promising Preclinical Antitumor Activity Across Several Cancers

The oral modified dysfunctional tyrosine SM-88 has demonstrated antitumor activity and appears to broadly effect the immune dynamics of the tumor microenvironment, according to early data from 2 xenograft studies presented during the 2020 AACR Virtual Annual Meeting II.

“Single-agent SM-88 displays antitumor effects in both athymic nude mice and immune-competent mice. [The agent] appears to have a broad impact on the immune dynamics of the tumor microenvironment, including reductions in M2 macrophages, CD4+ T cell populations, and T-reg populations,” said first author Alexander Vandell, PharmD, PhD, vice president of Clinical Research at Tyme Inc, during a poster presentation. “SM-88 also appears to disrupt autophagy or similar cellular processes that are important for cancer cell survival in addition to increasing reactive oxygen species (ROS) [production].”

SM-88 is a chemically-modified racemic amino acid therapy that was designed to leverage the cancer’s unique consumption of nonessential amino acids and disrupt various cancer mechanisms, according to Vandell. Clinical efficacy data that have been reported to date with the agent have been very encouraging, indicating broad anticancer activity spanning 15 different tumor types. This efficacy may be due to the agent’s impact on cancer via several mechanisms such as immune modulation, alterations in autophagy, and ROS induction, write Vandell and colleagues.

The data presented during the meeting were part of a comprehensive in vitro and in vivo experimental program that is currently underway. The goal of the program is to develop a better understanding of the agent’s mechanism of action and better characterize its anticancer effects. Specifically, investigators are examining in vitro effects on apoptosis, viability, ROS induction, autophagy, migration and invasion, cell cycle changes, and protein synthesis. Through the use of in vivo xenograft models, investigators are exploring anticancer effects of the agent as well as identifying its potential for immune modulation.

In the initial study, investigators used an HCT-116 colon cancer xenograft model to evaluate a range of SM-88 doses, according to Vandell. For this research, investigators subcutaneously implanted HCT-116 cells in female athymic nude mice. Once tumors reached 50 mm³-100 mm³, the mice were then given either the vehicle alone, 81 mg/kg of SM-88 daily, 162 mg/kg of SM-88 daily, or 324 mg/kg of SM-88 daily (n = 11, per group).

Results showed that the majority of the mice were still alive by the 19th day of the study across the arms. No deaths were reported in the vehicle arm or 81 mg/kg of SM-88 cohort. One death was reported in the 162 mg/kg SM-88 cohort, and 2 deaths were reported within the 324 mg/kg SM-88 cohort. Notably, however, mice that were given a 324 mg/kg daily dose of SM-88 experienced a significant reduction in tumor growth compared with mice that were given the vehicle alone by day 19 post treatment initiation (P <.05). No differences in body weight changes were observed among the different treatment cohorts.

“While there were some modest reductions in tumor size observed with the lower doses [of SM-88], the most meaningful antitumor effects were seen at the highest dose studied,” noted Vandell.

In the second study, investigators used a Pan02 pancreatic cancer syngenic xenograft model to examine immune alterations, according to Vandell. For this study, C57BL/6 mice were given either vehicle alone, either a daily dose of SM-88 methyl-ester (SM-88 ME), which contains the same active moiety as SM-88, but with better solubility characteristics, at 25 mg/kg via intraperitoneal (IP) injection or 75 mg/kg per day administered via IP injection (n = 10 per group). Treatment began on day 0, and on day 4 of the experiment, the mice were each subcutaneously implanted with Pan02 cells.

Results showed that the Pan02 tumor growth in mice that received the daily 75 mg/kg dose of SM-88 ME was significantly reduced versus those that received the vehicle alone by day 34 post treatment initiation. By day 34, the end of treatment, all mice (n = 30) were reported to be alive with no differences in body weight changes observed among all treatment groups. A higher number of mice who were given SM-88 were able maintain tumors below a volume of 500 mm³; this was reported along with an indication of a dose-related effect with the agent. “Twice as many tumors from the 75mg/kg dose group remain below this threshold compared with the control group,” said Vandell.

In order to properly analyze the effect of SM-88 on the tumor immune microenvironment, investigators utilized flow cytometry using a 16-marker panel to characterize immune populations in 5 randomly-selected subjects with Pan02 tumors. Results from this immune analysis indicated that treatment with SM-88 decreased intratumoral CD4+ T-cell populations, but preserved populations of CD8+ T cells; overall, this lead to decreases in the CD4+/CD8+ ratio, which was P = 0.015 for the daily SM-88 dose of 75 mg/kg versus the vehicle. SM-88 was also found to lower intratumoral T-reg populations, thereby potentially reducing immunosuppressive signaling within the tumor. Some smaller, but noteworthy, post-therapy increases in intracellular B-cell populations were observed.

Additional results indicated preservation of M1 polarized macrophage populations with SM-88 exposure; these populations are known to have antitumor effects. Additionally, treatment with the agent was found to lead to a reduction in M2 polarized macrophage populations, which are known to promote tumor growth. Based on these findings, investigators posit that these alterations within the M1/M2 ratio could help to suppress, and even reduce, tumor growth. 

Finally, examined the potential of the agent to increase the production of ROS through the utilization of 4 cancer cell lines: 2 pancreatic cancer (Pan002 and PANC1) and 2 breast (4T1 and MCF-7). Results demonstrated dose-dependent increases in ROS production 24 hours post-SM-88 ME exposure.

With regard to cellular effects, investigators evaluated the potential effect of SM-88 on autophagy in 3 cell lines: 2 pancreatic cancer (Pan02 and PANC1) and 1 ovarian cancer (HeLa). “We started with the expression of LC3B protein and we paired this with a marker, P62, which is normally inversely expressed during autophagy and we noticed that LC3B protein expression increased in several cell lines with SM-88 exposure, suggesting either a potential stimulation of autophagy or an impairment of a later phase of autophagy,” explained Vandell. “Also, P62 remained high, or [it] increased with the dose [of the agent] and that may suggest a disruption of the autophagy.”

Exposure to SM-88 ME led to a dose-dependent increase in LC3B expression at 24 hours in PANC1 and HELa cells. Specifically, exposure to 1mM of SM-88 ME appeared to encourage an increase in LC3B expression that started at least 6 hours after treatment initiation in both Pan02 and PANC1 cells. However, p62 expression did not appear to be impacted by the therapy. Collectively, these findings indicate that SM-88 had a notable effect on autophagic mechanisms. As such, further research efforts are being made to determine whether these changes in LC3B expression are caused by autophagy induction or blockade autophagosome and lysosome fusion.

Based on these data investigators plan to further explore the SM-88 in combination with chemotherapy, targeted therapies, and immunotherapies. Research efforts are also being dedicated to examining effects of the agent as well as potential biomarkers of sensitivity to SM-88.

“Our research efforts will initially focus on determining more details around autophagy interactions, including what steps in the process may be affected [by SM-88 exposure], as well as similar pathways like mitophagy, gaining a greater detail on the immune alterations with SM-88 and understanding potential signaling pathways that might be involved so immunotherapy combinations with the agent can be explored,” concluded Vandell. “We also will examine additional dose optimization and testing with other xenograft models.”

Reference

Vandell A, Eckard J, Hoffman S, et al. In vitro and in vivo anticancer effects of D/L-alpha-metyrosine (SM-88), a novel metabolism-based therapy. Presented at: AACR Annual Meeting. June 22-24, 2020. Abstract 5998. bit.ly/3fUGGjg.