[Cardiotoxicity of Anti-PD-L1 Antibody and the Effect of Levothyroxine 
in Attenuating the Related Mortality in Mice].

BACKGROUND

Immune checkpoint inhibitors (ICIs) such as antibodies against programmed cell death 1 (PD-1) and programmed cell death ligand 1 (PD-L1), have shown remarkable efficacies in many subtypes of cancers. However, ICIs may also cause severe immune-related adverse events in the recipient patients. Recently, ICI-associated myocarditis have been reported in hundreds of patients worldwide, with a mortality rate of approximately 50% in these cases. This study aims to recapitulate the cardiotoxicity and explore the detoxicifying approaches to attenuate mortality caused by PD-1/PD-L1 inhibitors in healthy mice.

METHODS

Six to eight-week-old C57BL/6 mice were inoculated with anti-PD-1 antibody (12.5 μg/g every 5 days for 6 injections), anti-PD-L1 antibody (10 μg/g once a week for 6 weeks), anti-PD-L1 antibody (with the same dosage described above) in combination with levothyroxine (0.25 μg/g, intraperitoneally injected half an hour before anti-PD-L1 antibody injection), or isotype control immunoglobulin IgG (10 μg/g once a week for 6 weeks). The ejection function of the hearts was detected by echocardiography, body temperature and blood pressure were detected by Mouse MonitorTM and non-invassive blood pressure minotor, and serum free thyroxine concentration was detected by The enzyme linked immunosorbent assay (ELISA).

RESULTS

PD-L1 was expressed at different levels by the cardiomyocytes of the mice. The isotype control immunoglobulin and anti-PD-1 antibody did not cause death of the mice. The 12 mice receiving 3-6 injections of anti-PD-L1 antibody showed a significant increase in the heart-to-tibial ratio and cardiomyoctye degeneration, hyalinization and extravascular inflammatory cell infiltration. In addition, the serum thyroxine was mardedly decreased to 1/3 of that in the control group mice, and the blood pressure and body temperature were abnormally decreased in mice upon treatment with PD-L1 blockade. Eight of the 12 (66.7%) mice died from multiple intravenous injection of anti-PD-L1 antibody.Intraperitoneal injection of levothyroxine 30 min before the injection of anti-PD-L1 antibody significantly attenuated the mortality rate of the anti-PD-L1 antibody-treated mice.

CONCLUSIONS

The anti-PD-L1 antibody is cardiotoxic and lethal, and levothyroxine is able to rescue the mice from this immune checkpoint inhibitor-caused mortality.