Background: Rapamycin (RAPA), is an immunosuppressive agent that inhibits the mechanistic target of Rapamycin (mTOR), the kinase subunit of two mTOR- containing complexes (mTORC1 and mTORC2). While RAPA selectively targets mTORC1, its effect on mTORC2 is less consistent. By contrast, ATP-competitive mTOR kinase inhibitors (TORKinibs) suppress the catalytic function of both complexes. Although mTORC2 has been reported to play an important role in B cell and follicular B helper T cell (Tfh) homeostasis, little is known about the immunosuppressive potential of TORKinibs.

Methods: A 9-d course of the TORKinib AZD2014 (10 mg/kg ip twice daily) or RAPA (1mg/kg ip daily) was administered to recipients of MHC-mismatched heart allografts (BALB/c to B6). Graft survival was assessed by abdominal palpation and rejection confirmed histologically. Immune cell subsets in spleen and cytokine production by host lymphocytes were analyzed by mAb staining and flow cytometry. Donor-specific alloantibody (DSA) titers were measured in serum 21 d post-transplant.

Results: Both AZD2014 and RAPA significantly prolonged graft survival (median survival time 25 d for AZD and 100 d for RAPA vs 9.5 d for control; both p<0.001). This effect was associated with reduced graft mononuclear cell infiltration, reduced Tfh (1.38×10⁵ vs 6.93 x10⁵; p=0.08) and B cells (59.46×10⁶ vs 93.43×10⁶; p=0.01) and an increased ratio of regulatory T cells (Treg) to effector memory T cells (Tem) in spleens (1.8 vs 1.1; p=0.055) of AZD -treated mice vs controls at 7 d post-transplant. However, 21 d post-transplant, i.e. 10 d after drug withdrawal, marked differences were observed between RAPA and AZD-treated mice. Specifically, the number of Tfh (1.65 x 10⁶ vs 0.37 x 10⁶; p=0.001) and B cells (235.5 x 10⁶ vs 127.6 x 10⁶; p=0.04) in the spleen and titers of IgG1 (1:10935 vs 1:1845; p<0.001) and IgG2A (1:2445 vs 1:135; p=0.07) DSA were lower in mice given RAPA. While the ratio of Treg to Tem was maintained after withdrawal of RAPA, the ratio decreased in AZD-treated mice (1.68 vs 1.20; p=0.02)

Conclusions: The TORKinib AZD2014 is an effective anti-rejection agent. However, unlike RAPA, its ability to regulate alloreactive T and B cell responses, DSA production and graft survival, is not sustained after its withdrawal. These differences may be explained by differences in pharmacokinetics or by distinct effects of each inhibitor on mTORC2 function.

CITATION INFORMATION: Fantus D, Ono Y, Dai H, Yokota S, Yoshida O, Thomson A. Differential Effects of the Novel ATP-Competitive mTOR Inhibitor (TORKinib) AZD2014 and Rapamycin on Alloimmunity and Transplant Survival. Am J Transplant. 2017;17 (suppl 3).

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