**3.3. Antimetabolite purine analogues**

1c9-dihydroxy-ciclosporin, 1-carboxy-ciclosporin) are toxic contributing to the nephrotoxic and hepatotoxic properties of the parent compound [33, 34]. Consequently, high CYP3A activity increases the rate of ciclosporin metabolism and decreases the immunosuppressive effect, which requires dose modification [35]. However, high CYP3A activity also increases the toxic metabolite formation and the risk of nephrotoxicity and hepatotoxicity. Therefore, immunosuppressive strategy must consider the blood concentrations of both ciclosporin and the toxic metabolites, especially if they are accompanied with symptoms indicating nephrotoxicity or

414 Organ Donation and Transplantation - Current Status and Future Challenges

Immunosuppressive properties of tacrolimus are similar to ciclosporin; however, for the same pharmacological effect, significantly lower blood concentration of tacrolimus is required than that of ciclosporin. Tacrolimus, the 23-membered macrocyclic lactone, is converted by demethylation, hydroxylation, and ring rearrangement to at least 15 metabolites, and only a minor proportion of tacrolimus dose is eliminated as unchanged parent drug [36]. Metabolism of tacrolimus leads to the inactivation of the molecule, except for the major 13-*O*-demethyl and the minor 31-*O*-demethyl metabolites. The 13-*O*-demethyl-tacrolimus possesses some immunosuppressive effect; however, it is about one tenth as active as tacrolimus, whereas the 31-*O*-demethyl metabolite displays an immunosuppressive activity comparable to tacrolimus [37, 38]. On the other hand, high blood concentration of 15-*O*-demethyl-tacrolimus metabolite has been reported to be associated with nephrotoxicity and myelotoxicity and with higher incidence of infections [39]. Similarly to ciclosporin, tacrolimus is metabolized by CYP3A enzymes, anticipating great interindividual and intraindividual differences in pharmacokinetics of tacrolimus: (1) CYP3A activity of enterocytes contributes to the first-pass metabolism of tacrolimus; (2) substantial interindividual differences in hepatic CYP3A activity result in great variability in the rate of tacrolimus metabolism, which requires continuous drug monitoring and dose modification primarily in the early postoperative period; (3) concomitant treatment with CYP3A inhibitors is the potential source of metabolic drug interactions; (4) genetic polymorphisms of CYP3A5 also contribute to the high interindividual variability. Since the relative contribution of CYP3A5 to tacrolimus biotransformation is significantly higher than that of CYP3A4 [40], the recipients carrying wild type *CYP3A5\*1* allele or transplanted with liver grafts carrying *CYP3A5\*1* are able to metabolize tacrolimus more rapidly than CYP3A5

The mTOR inhibitors prevent cell proliferation by blocking cell cycle progression from the G1-phase to the S-phase. The immunosuppressive activity is mediated *via* blocking mTOR protein kinases, resulting in inhibition of growth factor–mediated T-cell proliferation in response to IL-2 trigger [42]. Sirolimus is a 31-membered macrolide, whereas everolimus is a sirolimus derivative having a 2-hydroxyethyl chain substitution at position 40. Although the chemical structures of sirolimus and everolimus are similar to that of tacrolimus, the mechanism of action of mTOR inhibitors is distinct from calcineurin inhibitors, which allows the application of combination regimens. Additionally, the main advantages of mTOR inhibitors are their nonnephrotoxic properties; therefore, mTOR inhibitors in combination with reduced dose calcineurin inhibitors can augment the calcineurin inhibitor–induced nephro-

hepatotoxicity.

nonexpressers [35, 41].

toxicity [43–45].

**3.2. mTOR (mammalian target of rapamycin) inhibitors**

One of the oldest agents with immunosuppressive activity introduced for kidney transplant recipients was the purine analogue 6-mercaptopurine, which acts by inhibiting purine nucleotide synthesis and, as a consequence, cell proliferation. The prodrug of 6-mercaptopurine, azathioprine with more favorable side-effect profile was later introduced to prevent rejection. Azathioprine is converted to 6-mercaptopurine by nonenzymatic cleavage of the thioether in enterocytes and hepatocytes or in erythrocytes. The major active metabolites, 6-thioguanine nucleotides, are formed *via* 6-thioinosine monophosphate in natural purine synthetic pathways. Inhibition of cell proliferation is mediated by incorporation of the thiopurine nucleotide analogues into DNA (and RNA), causing DNA damage [48]. 6-Mercaptopurine, independently from either direct administration or production from azathioprine, undergoes metabolic inactivation by xanthine oxidase and thiopurine S-methyl transferase and is excreted in the urine, leaving less parent compound available to form thiopurine nucleotides [49]. Due to genetic polymorphism, the thiopurine S-methyl transferase activity is highly variable in patients; namely, those subjects who carry one or two nonfunctional thiopurine S-methyl transferase alleles are unable to tolerate normal doses of azathioprine and can experience serious myelosuppression [50]. Therefore, genotyping assay is recommended before starting azathioprine therapy to identify high-risk patients, and dosage reduction or alternative therapy is recommended for these patients.

#### **3.4. Inosine monophosphate dehydrogenase inhibitors**

Mycophenolic acid is a selective inhibitor of inosine monophosphate dehydrogenase, which is responsible for *de novo* biosynthesis of guanosine monophosphate, one of the building blocks of DNA. Depletion of the guanosine pool in the cell arrests the lymphotic cell proliferation and suppresses the subsequent immune response triggered by allogenic transplanted organ [51]. In several rapidly dividing cells (e.g. enterocytes), an alternative salvage pathway exists for purine synthesis in addition to *de novo* synthetic pathway; however, lymphocytes seem to be dependent on the *de novo* pathway. Consequently, mycophenolic acid is able to selectively block proliferation of T- and B-cells. Mycophenolic acid is available as enteric-coated mycophenolate sodium and as mycophenolate mofetil ester prodrug that is extensively hydrolyzed to the active metabolite mycophenolic acid by carboxylesterases.

Mycophenolic acid is primarily metabolized by UDP-glucuronyl transferases (UGT1A7/8/9, UGT2B7), forming the major 7-*O*-mycophenolic glucuronide that is pharmacologically inactive and to the minor acil-glucuronide that has pharmacological activity comparable to the mycophenolic acid [52, 53]. The major proportion of the glucuronide conjugates is excreted in urine, whereas a smaller proportion that is eliminated *via* bile is metabolized by bacteria in the gut, and the deconjugated mycophenolic acid can be reabsorbed (enterohepatic circulation) [54]. Furthermore, in patients' blood and urine, a minor demethylated metabolite (6-*O*-demethylmycophenolic acid) was also detected that was proved to be produced by CYP3A enzymes.

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,

Metabolic Drug Interactions with Immunosuppressants http://dx.doi.org/10.5772/intechopen.74524 417

**4. Significant metabolic drug interactions with immunosuppressants**

Transplant recipients' immunosuppressive therapy is often a multidrug therapy, primarily in the early postoperative period, which constitutes a challenge for clinicians to consider the complexity of drug interactions. Due to the fact that the metabolism of immunosuppressants with low molecular weight is catalyzed by the same enzymes (CYP3A4 and CYP3A5), the blood concentrations, elimination half-lives, and consequently, the efficacy or toxicity of certain immunosuppressant agents are expected to be modified during concomitant treatment. Therefore, during multidrug therapy or during withdrawal of any of the immunosuppressive drugs, special attention is required for optimal dosing for therapeutic concentrations. Each modification in immunosuppressive regimens can lead to changes in blood concentration of a drug (**Table 2**).

Calcineurin inhibitors are often applied in combination with mTOR inhibitors. Since both mTOR inhibitors and calcineurin inhibitors are substrates of CYP3A enzymes and can inhibit CYP3A activities, reduction of calcineurin inhibitor doses is recommended. Standard doses of ciclosporin were observed to decrease the clearance of sirolimus or everolimus more substantially than the doses of tacrolimus [45]. The major drawback of calcineurin inhibitor therapy is the risk of nephrotoxicity which appears to be dose dependent. The combination of low calcineurin inhibitor doses with mTOR inhibitors was found to be beneficial regarding retaining low rejection rates and lowering the risk of nephrotoxicity [44, 63]. To avoided renal dysfunction, the complete substitution of calcineurin inhibitors for mTOR inhibitors was attempted; however, the substitution showed an increase in graft failure in patients treated with merely mTOR

Corticosteroids have been demonstrated to induce the expression of the efflux pump transporter ABCB1 (P-glycoprotein) playing a main role in intestinal drug absorption and of CYP3A enzymes responsible for the metabolism of the majority of drugs [18, 65]. Therefore, the concomitant treatment of calcineurin inhibitors or mTOR inhibitors with corticosteroids can be expected to decrease the blood concentrations of tacrolimus/ciclosporin or of sirolimus/everolimus. Although the evidence for clinically significant interactions between corticosteroids and ciclosporin or mTOR inhibitors is limited, clear clinical effect of corticosteroids on tacrolimus exposure has been demonstrated [66, 67]. This also implies that dose reduction or cessation of corticosteroids leads to an increase in blood concentrations of tacrolimus, requiring dose

further development of sotrastaurin in transplantation has been halted.

**4.1. Combined immunosuppressive therapy**

inhibitors [64].
