The Efficacy of Rabbit Antithymocyte Globulin (Thymoglobulin^®) with Cyclosporin As First-Line Treatment in Aplastic Anemia,

Antithymocyte globulin (ATG) is the drug of choice for immunosuppressive therapy (IST) in patients with aplastic anemia (AA) unsuitable for hematopoietic stem cell transplantation. The standard ATG preparation in AA had been horse ATG because of the larger experience and the results already reported with this preparation. Due to the unavailability of the horse ATG since 2006, rabbit ATG became the only available ATG preparation in Korea. But, there are only limited data about the therapeutic efficacy of rabbit ATG as first-line IST in AA.

We retrospectively investigated the outcome of 58 evaluable patients among 62 patients with AA treated with IST using rabbit ATG as front line between March 2006 and April 2010 at our institution. 70.7% of enrolled patients were very severe (n=18) or severe AA (n=23). All patients received rabbit ATG (Thymoglobulin^®, 2.5mg/kg per day for 5 days) with methylprednisolone and cyclosporine A. Response rate (RR) was assessed at 3, 6, 9, 12 and 18 months after IST.

After IST, overall RR was 27.8%, 50.8%, 52.8%, 52.8% and 56.7% after 3, 6, 9, 12 and 18 months, respectively. Complete response (CR) rates were 0.8%, 1.8%, 5.6%, 9.6% and 21.2% after 3, 6, 9, 12 and 18 months, respectively. Median time to achieve partial response (PR) and CR were 93 (range; 12∼977 days) and 381 (range; 12–614) days. Among 31 responders, 10 patients (32.3%) relapsed. Median time between response and relapse were 396 days (range; 254∼681 days).

Estimated overall survival and failure free survival at 3 years from ATG treatment were 85.8% and 42.8%, respectively. Age (>45) at the use of ATG was an independent predictor of overall survival and overall response (p=0.033 and 0.027) in univariate analysis. Other factors such as disease severity, pre-ATG hematological parameters (absolute lymphocyte count and absolute reticulocyte count) were not associated with overall survival and failure free survival.

These data indicate that rabbit ATG was as effective as horse ATG based on other recently published data. But, the response time of rabbit ATG treatment is longer than that of horse ATG treatment. Especially, more than half of patients who achieved CR required time over 1 year after ATG treatment. Rabbit ATG can be effective IST regimen comparable to horse ATG, but it takes longer time to achieve sufficient response.

No relevant conflicts of interest to declare.