Safety (toxicity), pharmacokinetics, immunogenicity, and impact on elements of the normal immune system of recombinant human IL-15 in rhesus macaques

Abstract

IL-15 uses the heterotrimeric receptor IL-2/IL-15Rβ and the γ chain shared with IL-2 and the cytokine-specific IL-15Rα. Although IL-15 shares actions with IL-2 that include activation of natural killer (NK) and CD8 T cells, IL-15 is not associated with capillary leak syndrome, activation-induced cell death, or with a major effect on the number of functional regulatory T cells. To prepare for human trials to determine whether IL-15 is superior to IL-2 in cancer therapy, recombinant human IL-15 (rhIL-15) was produced under current good manufacturing practices. A safety study in rhesus macaques was performed in 4 groups of 6 animals each that received vehicle diluent control or rhIL-15 at 10, 20, or 50 μg/kg/d IV for 12 days. The major toxicity was grade 3/4 transient neutropenia. Bone marrow examinations demonstrated increased marrow cellularity, including cells of the neutrophil series. Furthermore, neutrophils were observed in sinusoids of enlarged livers and spleens, suggesting that IL-15 mediated neutrophil redistribution from the circulation to tissues. The observation that IL-15 administration was associated with increased numbers of circulating NK and CD8 central and effector-memory T cells, in conjunction with efficacy studies in murine tumor models, supports the use of multiple daily infusions of rhIL-15 in patients with metastatic malignancies.

Figure 1

Pharmacokinetics of rhIL-15 assessed in rhesus macaques. (A) rhIL-15 pharmacokinetics were evaluated in rhesus macaques, with one set of animals receiving 12 infusions of 10 (•), 20 (■), and 50 (▴) μg/kg/d of rhIL-15 for 12 days, followed within 24 hours by killing and necropsy. IL-15 pharmacokinetics in this group were evaluated at 10, 30, 60 minutes, 4 hours, and 24 hours after the initial rhIL-15 administration. The mean terminal half-time was 1.11 ± 0.25 hours for the 10 μg/kg/d group, 1.10 ± 0.16 hours for the 20 μg/kg/d group, and 0.91 ± 0.11 hours for the 50 μg/kg/d group. (B) Other groups of animals each received the same doses and dosing schedule but were killed and necropsied at day 48 after a 36-day recovery period. In this set of animals, IL-15 pharmacokinetics were evaluated in the same schedule as in panel A after the 12th dose of rhIL-15. There was no statistical difference in the parameter values between day 1 (first infusion) and day 12 (final infusion) by Student t test, with the exception of the terminal slope of the 50 μg/kg/d animal. (C) A single additional animal received a 200 μg/kg/d dosage. At 24 hours after the IL-15 infusion, there was 270 pg/mL of rhIL-15 retained in this animal, a value only 0.04% of the ∼ 6 × 10⁶ pg/mL at the 10-minute time point. Therefore, only in this latter animal was there any evidence for a prolonged persistence of rhIL-15 in the serum.

Figure 2

rhIL-15 administration reduces the circulating absolute neutrophil numbers in rhesus macaques. Vehicle diluent or rhIL-15 at 10, 20, and 50 μg/kg/d was administered daily for 12 days to rhesus macaques. The circulating neutrophil numbers were determined before therapy and on days 8 and 13 for animals killed on day 13, and before therapy and on days 8, 13, 15, 22, 29, and 48 for those killed on day 48. Three of 6 animals receiving 20 μg/kg/d and 3 of 6 animals receiving 50 μg/kg/d of rhIL-15 had a grade 3/4 reduction in circulating neutrophil numbers. These reductions were transient, with a return toward normal levels by 72 hours after cessation of rhIL-15 administration.

Figure 3

rhIL-15 administration is associated with hypercellular marrow and a reduction in adipocytes. Shown is the impact of rhIL-15 on the bone marrow examination of animals receiving vehicle diluent control or rhIL-15 at 50 μg/kg/d for 12 days. Left panel is the vehicle control bone marrow at day 44. Center panel is the bone marrow of animals receiving 50 μg/kg/d for 12 days determined at necropsy at day 13 within 24 hours of the last rhIL-15 administration showing marrow hyperplasia with a reduction in adipocytes. Right panel is the bone marrow examination of rhesus macaques receiving 50 μg/kg/d for 12 days studied at necropsy at day 44, 32 days after the last rhIL-15 administration, showing a return to a normal marrow pattern.

Figure 4

rhIL-15 administration is associated with an enlarged liver with normal hepatocytes, but with the presence of leukocytes including lymphocytes and neutrophils in the liver sinusoids. Left panel is the liver histology of an animal receiving vehicle control for 12 days with a necropsy on day 13. Center panel is the liver histology of an animal receiving 50 μg/kg/d for 12 days, with necropsy at day 13, showing normal hepatocytes but the presence of leukocytes including lymphocytes and neutrophils in the liver sinusoids. Right panel is the liver histology of an animal receiving 50 μg/kg/d of rhIL-15 for 12 days with necropsy at day 44, showing a return toward a normal pattern.