**3.5. Corticosteroids**

At the beginning of transplantation history, glucocorticosteroids were the primary immunosuppressive agents in the rejection prophylaxis strategy, and nowadays, they are still the first-line agents for treatment of graft rejection. The high-dose glucocorticoids given in peritransplantation are tapered to low doses in the maintenance phase, aiming the steroid-free immunosuppression regimens because of serious adverse effects of glucocorticoids developing in long-term therapy. Acute rejection is generally treated with methylprednisolone, whereas the maintenance therapy applies either methylprednisolone or prednisone. Corticosteroids activate the cytosolic glucucorticoid receptor and modulate several cellular functions, including transcription of genes involved in proliferative and inflammatory processes. The activated receptor inhibits the transcription of NF-kB and activator protein 1 dependent genes, including proinflammatory cytokines (**Figure 1**). This process leads to the depletion of T-cells and macrophage dysfunction [55].

Regioselective and stereospecific hydroxylation of corticosteroids at several positions (at carbon 2, 6, 7, 15, 16, and 21) are catalyzed by CYP3A enzymes. Additionally, dual effect of corticosteroids on CYP3A enzymes has been demonstrated: (1) corticosteroids can competitively inhibit the function of CYP3A [56], and (2) they can induce CYP3A transcription. Activated glucocorticoid receptor upregulates the expression of nuclear receptors (PXR and CAR) that are involved in transcriptional regulation of CYP3A genes. Moreover, the proximal promoter region of CYP3A4 gene contains glucocorticoid responsive element, which directly binds activated glucocorticoid receptor [18, 57]. As a consequence of increased expression and activity of CYP3A enzymes, metabolic drug interactions can be expected upon concomitant treatment with drugs that require CYP3A activity for their metabolism.

#### **3.6. Novel investigational immunosuppressant agents**

Although calcineurin inhibitor–based immunosuppression efficiently prevents rejection, adverse reactions of ciclosporin and tacrolimus, primarily nephrotoxicity, prompt the discovery of novel agents with immunosuppressive activity [58]. Two investigational agents with low molecular weight should be mentioned: voclosporin and sotrastaurin. Voclosporin, a next-generation calcineurin inhibitor, is an analogue of ciclosporin with a single carbon extension added to the amino acid-1 of ciclosporin. Voclosporin displays higher binding affinity to cyclophillin A than ciclosporin leading to more potent inhibition of calcineurin [59]. Furthermore, it has a favorable safety property that it appears to be less toxic than currently available calcineurin

inhibitors. Similarly to ciclosporin, voclospiron is a substrate for CYP3A enzymes, anticipating pharmacokinetic/metabolic drug interactions with those agents that interact with ciclosporin as well [60]. However, voclosporin is no longer pursed in transplantation. Sotrastaurin is protein kinase C inhibitor that effectively inhibits IL-2 production with the mechanism different from calcineurin or mTOR inhibition. Although sotrastaurin displayed some potential in preventing allograft rejection in animal studies, high efficacy and safety failure rate were observed in clinical trials involving kidney and liver transplant patients [61, 62]. Therefore, further development of sotrastaurin in transplantation has been halted.
