**5. Regulatory aspects and final conclusions**

#### **5.1. Clinical practice recommendations**

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

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

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

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

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

polymorphism producing a premature STOP codon or a relevant aminoacid change.

patients to the occurrence of CNI-related nephrotoxicity [38].

were found according to the same SNP in the recipient.

296 Current Issues and Future Direction in Kidney Transplantation

the liver but also in the kidney.

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, CYP3A5 rs776746, has reached some kind of clinical recommendations. These have not been adopted by any of the regulatory agencies FDA nor EMA, but they already have a strong evidence as to be considered by expert doctors in the area.

**•** Quantify publication bias (as many studies fail to be published because their results are not

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

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

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**•** Help to document the approval of the use of interventions by regulatory agents and also

Conducting such studies is cumbersome, it is really time-consuming, requires complex methodological knowledge and its performance is not free of trouble. The main difficulties are the presence of a small number of previously existing studies, the fact that the selected studies to be analysed are usually very heterogeneous and difficult to be combined, and in many of them the necessary information is absent or with low methodological quality. However, meta-

However, we must be careful as the name "meta-analysis" does not ensure a quality review and readers should critically evaluate it before accepting its results, for which there are currently accessible guides [47, 48]. Its validity largely depends on the quality of the included studies and the absence of bias in its execution [49]. The studies analyzed in the meta-analysis are mostly randomized trials, which are those that offer the best evidence, but there are scenarios where the information comes only from observational studies [50], as studies on etiological hypotheses or adverse events. This represents a challenge as this type of design has a higher risk of bias and lack of essential information for the integration of studies [51]. Furthermore, the inclusion of studies with a large heterogeneity or variability between them, hinders the results interpretation [52], requires the knowledge of statistical tools for proper interpretation [53] and one must know that it is a limitation for the applicability of the results. Meta-analysis is a retrospective process, so it is susceptible to errors of this type of design. It could have biases in any of its stages: in the search and selection of studies, analysis and

The meta-analysis is the highest level of evidence and summarizes the studies available about a particular matter in a reliable way. Its implementation has its difficulties and limitations, so methodological rigor is required to help reduce the risk of bias and a critical and cautious view

As far as we know, two meta-analyses have been published regarding clinical implications of CYP3A5 and CNIs in renal transplantation. One is about tacrolimus [54] and its conclusion agrees with the data explained about CYP3A5 expressers/non-expressers dose requirements. The other one deals with cyclosporine [55], and also concludes that there is an association between our SNP and cyclosporine dose-adjusted concentration, where patients carrying \*3/\*3 genotype will require a lower dose of the drug to reach target levels, compared with \*1/

Calculate the sample size needed for future clinical trials about a similar topic.

significant) [45, 46].

synthesis of information.

of its results.

\*1 or \*1/\*3 carriers.

**•** Provide evidence to generate new hypotheses.

analysis studies are low-cost and have high impact.

expand the knowledge to academics.

**•** Decide whether further clinical trials are needed on the subject.

The 3rd European Science Foundation- University of Barcelona (ESF-UB) Conference in Biomedicine on Pharmacogenetics and Pharmacogenomics, held in June 2010 in Spain, published a summary of their practical recommendations [43] which include the explained tacrolimus results. They recommend CYP3A5 rs776746 genotyping prior to grafting as it could help to reach steady state plasma tacrolimus concentrations earlier, and therefore prevent overdose (risk of nephrotoxicity) or underdose (risk of acute graft rejection). This recommen‐ dation is mainly based on Thervet's publication [23] and suggests the introduction of tacroli‐ mus at 0.15 mg/kg/day when the recpient's genotype is \*3/\*3, at 0.20 mg/kg/day when it is \*3/ \*1, and at 0.25 mg/kg/day when it is \*1/\*1; always taking into consideration that the patients will also require the regular TDM.

The Dutch Pharmacogenetics Working Group Guideline from the Royal Dutch Pharmacists Association have also evaluated therapeutic dose recommendations for tacrolimus based on our CYP3A5 SNP [44] and have found evidence to support an interaction between the drug and the gene. However, they do not make dosing recommendations adducing that in dutch transplantation hospitals, the tacrolimus dose is titrated in response to TDM.
