**4.2. Pharmacogenetic examples in renal transplantation**

Moreover, one of the main problems of pharmacogenetic studies is the difficulty to recruit the number of patients needed to achieve sufficient statistical power and demonstrate conclusively the existence of significant clinically relevant differences according the different genotypes, that is, according to the different alleles or variants of a polymorphic site. This is due to the uneven distribution of allele frequencies in the population, which makes it difficult to collect a large enough number of individuals to study minor genotypes. Furthermore, the distribution of allelic frequencies in some genes varies according to ethnicity so, for instance, the expected frequencies of each allele of a polymorphic site in the Caucasian population are not the same as in the Asian. The expected effects of each allelic variant are presumably the same, but the

Figure 1 shows, in summary, an integrator scheme with the best known genes encoding transporter proteins and enzymes involved in the metabolism of several drugs commonly used

**Figure 1.** Integrative scheme of pharmacogenetic genes related to transport and metabolism of immunosuppressive drugs for transplantation. (Adapted from ref. 13, ©Astellas Pharma S.A. y Master Line & Prodigio S.L.). The drawing shows a broad view of the location and influence of metabolic enzymes and transporters on the main immunosup‐ pressive drugs used in transplantation. The integrated view of many of the genetic factors that influence the achieve‐ ment of therapeutic and stationary blood levels, should allow a better interpretation of pharmacogenetic data and

ease for recruiting different genotype patients is not.

294 Current Issues and Future Direction in Kidney Transplantation

also, help improve the safe and effective use of medication.

in transplantation.

Pharmacogenetic information of immunosuppressants in renal transplantation is mainly related to Tacrolimus, Cyclosporine and Mycophenolic Acid. Sirolimus, Everolimus and Corticoids are also being studied but to a much lesser extent, so we will focus here on the most consolidated conclusion about the first three drugs.

The first two, being both Calcineurin Inhibitors (CNI), share their mechanism of action and so, share transporters, metabolism enzymes and targets and therefore, they also share pharma‐ cogenetic results in most of the cases. The fact that they are both subject of a controlled therapeutic drug monitoring, with "in some way" standardized blood measuring methods, has allowed the publication of many works dealing with correlations between drug levels and polymorphisms [14-23]. To a lesser extent, there are also many works correlating drug adverse effects with SNPs [24-28]. The therapeutic drug monitoring of mycophenolic acid is not as followed as for CNIs, as there is not such a clear consensus about the effects of different blood levels in the possible drug related toxicity. However, many efforts have also been done in the pharmacogenetic studies of this drug [29-36], as it is widely employed in combination with tacrolimus or cyclosporine.


The most consensuated genes regarding polymorphic effects on these three immunosuppres‐ sants are shown in table 2.

**Table 2.** Most studied SNPs related to Tacrolimus, Cyclosporine and Mycophenolic Acid in Renal Transplantation.

As shown in the table, even in these SNPs that are the most extensively studied, the clinical implications are not always well established. Many more other SNPs are currently under research consideration, being what we can call "candidates" to have a clinical meaning. Virtually, every polymorphism of a gene implicated in a drug's route of transport, metabolism or mechanism of action is a potential candidate to be investigated. Especially if the polymor‐ phism is known to have a biological consequence on the gene product as for instance, if it is a polymorphism producing a premature STOP codon or a relevant aminoacid change.

compared to those patients that were \*3. Moreover, there are also consistent data in pediatric

Practical Pharmacogenetics and Single Nucleotide Polymorphisms (SNPs) in Renal Transplantation

http://dx.doi.org/10.5772/54733

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**Figure 2.** Influence of the functional activity of glycoprotein-P (transporter in apical membrane) in the transport of tacrolimus (green stars) in the intestine epithelium. The diagram shows the different degree of drug absorption due to variations in ABCB1/MDR1 polymorphic site rs1045642. Individuals with TT variant have a decreased transporter activ‐ ity and hence greater absorption efficiency. CC variant causes more expulsion out of the cell, which decreases absorp‐

Just as a final remark, we cannot forget to mention the great importance of drug interactions. Although it is not the subject of this chapter, and we are not going to get into it, we just wanted to point here that drug interactions can mask even genetic variations in the clinical practice.

We have only seen, with a little bit of detail, two of the SNPs that could actually be influencing the pharmacologic treatment in renal transplantation. And with these two SNPs, only one,

renal transplant recipients [42].

tion. (Figure adapted from ref. 39)

**5. Regulatory aspects and final conclusions**

**5.1. Clinical practice recommendations**

Returning to the SNPs in table 2, we will pay attention now to ABCB1 and CYP3A5 most rele‐ vant results. In Figure 2, we can see a schematic example of what happens in the intestine epi‐ thelial cells, according to the SNP rs1045642 C>T (also known as 3435 C>T) in ABCB1 gene. This gene codes for glicoprotein P (gp-P)which is an adenosine triphosphate-dependent transport‐ er, that pumps many endogenous substances and also xenobiotics, as drugs, outside of the cell. It is specifically expressed in the intestine, liver and kidney, amongst others, and also in several types of leukocytes so it is postulated to function as a protective barrier by actively extruding different compounds out of the cell, into the gut lumen, bile or urine. The expression of ABCB1 in the kidney plays an important role in the renal elimination of metabolic waste products and toxins. It seems like after renal injury, ABCB1 expression is upregulated, which may represent an adpative response in the renal regeneration process [37]. Parallelly, it has been shown that treatment with CNI induces ABCB1 expression both *in vivo* and *in vitro*, which could serve to protect the kidney from the injurious effects of CNIs by facilitating their extrusion. If we add to this, the polymorphic influence shown in figure 2, we can better understand that a failure to ad‐ equately upregulate ABCB1 expression or a constitutively low expression in renal cells (as for instance due to 3435 TT variant), could lead to intrarenal accumulation of CNIs and predispose patients to the occurrence of CNI-related nephrotoxicity [38].

Specifically, in renal transplantation, it has been found a correlation between the genotype of donors TT at this SNP and cyclosporine nephrotoxicity [40], while no consistent relationships were found according to the same SNP in the recipient.

Regarding CYP3A5, there is more statistical evidence, especially regarding its impact on CNIs blood levels and these findings have led to some clinical recommendations, as we will see in the next paragraph. Inside the CYP 450 family, the CYP3A subfamily metabolizes more than 50% of all drugs that are currently in use [41]. CYP3A5 is expressed in the small intestine and the liver but also in the kidney.

One of the most relevant studies regarding CYP3A5 SNP rs776746 A>G (\*1 (A), \*3 (G)) is the one published by Thervet et al. in 2010 [23]. It is a prospective randomized clinical trial that demonstrates the usefulness of this SNP determination before the first tacrolimus dose in renal transplantation. In this study, the pharmacokinetic parameters were correlated with the recipients' genotype and two arms were constructed, one with the classical management of the patients, adjusting tacrolimus doses according to TDM; and the second arm, where the initial dose was chosen according to a previous genetic analysis to include the patients in "CYP3A5 expresser" or "CYP3A5 non-expresser" cathegories. The expressers were given an initial 0.25mg/kg dose and the non-expressers 0.15mg/kg. As a result, the genetically driven dosage was associated with an earlier obtention of tacrolimus concentrations inside the therapeutic range, also with fewer dose adjustments. Also, it was demonstrated that in the first arm, patients with genotype \*1 needed double tacrolimus dose to reach the target levels, as compared to those patients that were \*3. Moreover, there are also consistent data in pediatric renal transplant recipients [42].

**Figure 2.** Influence of the functional activity of glycoprotein-P (transporter in apical membrane) in the transport of tacrolimus (green stars) in the intestine epithelium. The diagram shows the different degree of drug absorption due to variations in ABCB1/MDR1 polymorphic site rs1045642. Individuals with TT variant have a decreased transporter activ‐ ity and hence greater absorption efficiency. CC variant causes more expulsion out of the cell, which decreases absorp‐ tion. (Figure adapted from ref. 39)

Just as a final remark, we cannot forget to mention the great importance of drug interactions. Although it is not the subject of this chapter, and we are not going to get into it, we just wanted to point here that drug interactions can mask even genetic variations in the clinical practice.
