Lenalidomide induces impressive response rates in patients with multiple myeloma. Unfortunately, one of the major side effects of this therapy is the increased risk of thromboembolic events (TEE) up to 17%. The reason for a higher risk of TEE associated with Lenalidomide is not known, but it has been found that aspirin is very effective in preventing TEEs induced by thalidomide and lenalidomide (Rajkumar, 2005). Cathepsin G is a serine proteinase present in the azurophilic granules of polymorphonuclear neutrophils (PMNs) and is a strong platelet aggregation agonist with potency similar to thrombin. In the current study we investigated the effect of the immunomodulatory derivatives of thalidomide (IMiDs) including Actimid and lenalidomide on cathepsin G in vitro and in vivo. To investigate the effect of IMiDs in vitro CD34+ hematopoietic progenitors were cultured in the presence of IL-3, IL-6 and SCF supporting the development of granulocytes and treated with Actimid or DMSO (control). RNA and protein were extracted for the analysis of cathepsin G. A significant up-regulation of cathepsin G (7.7 fold) was detected in oligonucleotide gene array analysis after 3 days of treatment with Actimid as compared to control group. These results were confirmed by RT-PCR, which showed a 3.7 and 7.6 fold mRNA-increase on day 6 and day 10 of culture compared to control. Next we analyzed the cathepsin G levels of patients (n=10) treated with lenalidomide before treatment, on day 15 of each cycle (1–4) as well as 1, 2, and 3 month after stop of lenalidomide treatment. We observed a continuous significant increase of the mRNA levels of PMNs over the course of treatment (baseline: 1 fold, cycle 2: 1.7 fold, cycle 3: 4.8 fold, cycle 4: 20.7 fold). These data were confirmed by measuring cathepsin G levels in patient serum by ELISA. Cathepsin G significantly increased from a baseline mean of 53 ng/ml to 77.5 ng/ml (cycle 2), 129.2 ng/ml (cycle 3) and 145.5 ng/ml (cycle 4). After stop of the lenalidomide treatment we observed a time dependent normalization to baseline levels of the cathepsin G with 61 ng/mL (1 month post treatment), 42.5 ng/mL (2 months post treatment), and 49.2 ng/mL (3 months post treatment). Our data show that IMiDs up-regulate the potent platelet aggregation activator cathepsin G in hematopoietic cells and thereby might contribute to the development of TEE in patients receiving IMiD treatment. Based on our data TEE prophylaxis should be continued for at least 1 more month after stop of therapy. For the first time our results provide an explanation why aspirin is effective in preventing TEE in patients receiving IMiDs. Further studies are needed to determine whether cathepsin G helps to predict TEE. Inhibition of cathepsin G might be a potential therapeutic target for preventing the hypercoaguable state induced by IMiD treatment.

Author notes

Disclosure:Employment: David Stirling is an employee of Celgene Corporation Research Funding: Dr. Lentzsch has received research funding from Celgene Corporation, but it did not include the research reported here.