**3.2.1 PK-PD model**

PK-PD mechanistic-model. Around 50-60% of patients with depression have an increased activity of the hypothalamic-pituitary-adrenal axis and altered its regulation by negative feedback. Escitalopram and other SSRI produce an up-regulation of CRH receptors in a dose-dependent manner that can be measured by Nuggent`s test and this can be used for dosage regimen calculation of escitalopram and other SSRIs.

Once the steady-state and a pharmacological response, ranging between the 20-80% of maximal effect, is reached we can estimate the optimal dose required to achieve cortisol values of 9.0 ± 2.1 mcg/dl, its Clinical Surrogate Endpoint, using equation 2 (Lozano et al 2008a, 2011a).

### **3.2.2 PK-PG model**

Metabolic studies for Escitalopram /Citalopram indicate that CYP3A4 and CYP2C19 are the major isozymes involved in N-demethylation of Escitalopram /Citalopram. The alleles

Biomarkers and Therapeutic Drug Monitoring in Psychiatry 165

Prolactin plasma levels are also high in several drug treatments, such as: (a) Those that are expected to increase extracellular 5-HT levels in the brain as L-tryptophan, and 5 hydroxytrytophan, (b) those that acting stimulating different types of 5HT receptors, and some, but not all, 5HT1a selective agonists. In all these cases, prolactin plasma level becomes a surrogate of the increase of serotonergic transmission in one or more regions of the brain. The blockade of this effect, by an antagonist drug of the 5-HT receptors, appropriate and specific, can serve, in turn, as a substitute for the ability of a drug for the functional antagonization of the receptor (Goddard 1993; Golden et al 1989; Murphy et al 1996;; Seibyl

Risperidone, an atypical antipsychotic with high affinity for dopamine receptors D2 and Serotonin 5-HT2a, produces a dose-dependent D2 receptor occupancy in the mesolimbic system, necessary for the control of the disease, while the blockade in the nigrostriatal would be responsible for extrapyramidal side effects and in the tuberoinfundibular of the

Mechanistic-model. Risperidone produces a blockade of D2 receptor in the tuberoinfundibular system causing a dose-dependent increase in prolactin that can be used for dosing it. Using the values of serum prolactin, obtained in the morning before administration of Risperidone and not before steady-state plasma levels , minimum concentration in "steady-state", are reached we can construct the dose-response curve of prolactin plasma level versus Risperidone dose, which fits a rectangular modified hyperbola, equation 5, and permit us to establish a maximum value of PRL= 80-90 (40-45) ng/ml and a minimum value of PRL= 40-50 (20-25) ng/ml, for the optimal dose of

84 90 *Risp PRL*

Risperidone is eliminated by metabolism mainly through the action of citochrome CYP2D6, and to a lesser extent through CYP3A4. Risperidone and its main metabolite, 9-OHrisperidone, constitute the "active fraction" with an elimination half-life of 6 and 24 hours, respectively, in extensive metabolisers, EM, and 20 and 30 h in the case of patients with phenotype "poor metabolizer" PM. The "clearance", Cl, for the active fraction of Risperidone is respectively of 5.0 and 13.7 l/h in EM subjects and 3.2 and 3.3 l/h in individuals PM

The CYP2D6 activity ranges considerably within a population and includes Ultrafast metabolizers, UM, Extensive metabolizers, EM, Intermediate metabolizers,IM, and Poor metabolizers, PM. Among these are fully functional alleles, alleles with reduced function and null, non-functional, alleles, which convey a wide range of enzyme activity, from no activity to ultrarapid metabolism of substrates. Null alleles of CYP2D6 do not encode a functional protein and have no detectable residual enzymatic activity, being responsibles for

2 2

*Risp* (5)

risperidone, in women and men, in parentheses (Lozano et al 2007, 2010c):

et al 1991; Seletti et al 1995; Silverstone & Cowen 1994).

**3.3.1 Risperidone** 

hiperprolactinemy. **3.3.1.1 PK-PD model** 

**3.3.1.2 PK-PG model** 

(Janssen Research Foundation 1994).

CYP2C19 \* 2, CYP2C19 \* 3 and CYP2C19 \* 17 CYP2C19 and their combinations lead to phenotypes PM and IM, obtaining from the application of the equation 2, the following equi-effective dose ratios:


Paroxetine, Fluoxetine and Sertraline, which are metabolized by the CYP2D6 whose activity ranges considerably within a population and includes Ultrafast metabolizers, UM, Extensive metabolizers, EM, Intermediate metabolizers,IM, and Poor metabolizers, PM. Among these are fully functional alleles, alleles with reduced function and null, non-functional, alleles, which convey a wide range of enzyme activity, from no activity to ultrarapid metabolism of substrates. Null alleles of CYP2D6 do not encode a functional protein and have no detectable residual enzymatic activity, being responsibles for the PM phenotype in homozygous and for IM phenotype in heterozygous, when to present (Zhou 2008), obtaining from the application of the equation 2, the following equi-effective dose ratios:

$$\frac{\text{DOSE} \text{ (CYP2D6 phenotype} : \text{EM})}{\text{DOSE} \text{ (CYP2D6 phenotype} : \text{IM)}} = 2 \quad \text{and} \quad \frac{\text{DOSE} \text{ (CYP2D6 phenotype} : \text{EM)}}{\text{DOSE} \text{ (CYP2D6 phenotype} : \text{PM)}} = 4$$

(Lozano et al 2008a).

#### **3.3 Antipsychotics**

Schizophrenia first described by Benedict Morel in 1853 and with a prevalence of 1% in the general population, represents a group of chronic and severe mental disorders. Although its origin is unknown, there is an increased dopaminergic activity in the mesolimbic pathway of the brain. This biochemical alteration is used as target for the drug treatment of this disease, being the blockade of dopamine D2 receptors in the limbic system important for the control of the psychotic symptoms and the blockade of 5HT2a receptor for the control of negative symptoms.

Despite the unknown action mechanism of antipsychotic drug, is known that all of them act on the dopaminergic system but their affinitiy for dopamin and serotonin receptors,D2 and 5-HT, are different, being necessary the blockade of these receptors for its pharmacological action, being the combined blockade of the receptors 5-HT2a and D2 a method that evidence has proposed for the treatment of schizophrenia (Horacek et al 2006).

Prolactin plasma level, PRL, reflects the tuberoinfundibular D2 blockade and can be used as a Biomarker or Surrogate Marker for drugs affecting dopaminergic system and, in general, for atypical antipsychotics which main action was the blockade of D2 receptors, since they produce a dose-dependent prolactin increase that can be used as a surrogate for D2 receptor occupation and to serve at the same time for the dose adjustment and/or, by extension, the absence of prolactin increase levels after antipsychotic treatment could be considered a surrogate of a decrease occupancy of the D2 receptor, as for Aripiprazole. Eventually, low occupancy of D2 receptors in the striatum can be used as Surrogate Marker for the likelyhood of motor side effects (Avrantis & Miller 1997; Kapur et al 2000; Meltzer 1985; Nordstrom & Farde 1998).

Prolactin plasma levels are also high in several drug treatments, such as: (a) Those that are expected to increase extracellular 5-HT levels in the brain as L-tryptophan, and 5 hydroxytrytophan, (b) those that acting stimulating different types of 5HT receptors, and some, but not all, 5HT1a selective agonists. In all these cases, prolactin plasma level becomes a surrogate of the increase of serotonergic transmission in one or more regions of the brain. The blockade of this effect, by an antagonist drug of the 5-HT receptors, appropriate and specific, can serve, in turn, as a substitute for the ability of a drug for the functional antagonization of the receptor (Goddard 1993; Golden et al 1989; Murphy et al 1996;; Seibyl et al 1991; Seletti et al 1995; Silverstone & Cowen 1994).

#### **3.3.1 Risperidone**

164 Biomarker

CYP2C19 \* 2, CYP2C19 \* 3 and CYP2C19 \* 17 CYP2C19 and their combinations lead to phenotypes PM and IM, obtaining from the application of the equation 2, the following

Paroxetine, Fluoxetine and Sertraline, which are metabolized by the CYP2D6 whose activity ranges considerably within a population and includes Ultrafast metabolizers, UM, Extensive metabolizers, EM, Intermediate metabolizers,IM, and Poor metabolizers, PM. Among these are fully functional alleles, alleles with reduced function and null, non-functional, alleles, which convey a wide range of enzyme activity, from no activity to ultrarapid metabolism of substrates. Null alleles of CYP2D6 do not encode a functional protein and have no detectable residual enzymatic activity, being responsibles for the PM phenotype in homozygous and for IM phenotype in heterozygous, when to present (Zhou 2008), obtaining from the application of the equation 2, the following equi-effective dose ratios:

Schizophrenia first described by Benedict Morel in 1853 and with a prevalence of 1% in the general population, represents a group of chronic and severe mental disorders. Although its origin is unknown, there is an increased dopaminergic activity in the mesolimbic pathway of the brain. This biochemical alteration is used as target for the drug treatment of this disease, being the blockade of dopamine D2 receptors in the limbic system important for the control of the psychotic symptoms and the blockade of 5HT2a receptor for the control of

Despite the unknown action mechanism of antipsychotic drug, is known that all of them act on the dopaminergic system but their affinitiy for dopamin and serotonin receptors,D2 and 5-HT, are different, being necessary the blockade of these receptors for its pharmacological action, being the combined blockade of the receptors 5-HT2a and D2 a method that evidence

Prolactin plasma level, PRL, reflects the tuberoinfundibular D2 blockade and can be used as a Biomarker or Surrogate Marker for drugs affecting dopaminergic system and, in general, for atypical antipsychotics which main action was the blockade of D2 receptors, since they produce a dose-dependent prolactin increase that can be used as a surrogate for D2 receptor occupation and to serve at the same time for the dose adjustment and/or, by extension, the absence of prolactin increase levels after antipsychotic treatment could be considered a surrogate of a decrease occupancy of the D2 receptor, as for Aripiprazole. Eventually, low occupancy of D2 receptors in the striatum can be used as Surrogate Marker for the likelyhood of motor side effects (Avrantis & Miller 1997; Kapur et al 2000; Meltzer 1985;

has proposed for the treatment of schizophrenia (Horacek et al 2006).

 DOSE CYP2C19 phenotype : EM <sup>4</sup> DOSE CYP2C19 phenotype : PM

 DOSE CYP2D6 phenotype : EM <sup>4</sup> DOSE CYP2D6 phenotype : PM

equi-effective dose ratios:

(Lozano et al 2008a).

**3.3 Antipsychotics** 

negative symptoms.

Nordstrom & Farde 1998).

 DOSE CYP2C19 phenotype : EM <sup>2</sup> DOSE CYP2C19 phenotype : IM and

 DOSE CYP2D6 phenotype : EM <sup>2</sup> DOSE CYP2D6 phenotype : IM and Risperidone, an atypical antipsychotic with high affinity for dopamine receptors D2 and Serotonin 5-HT2a, produces a dose-dependent D2 receptor occupancy in the mesolimbic system, necessary for the control of the disease, while the blockade in the nigrostriatal would be responsible for extrapyramidal side effects and in the tuberoinfundibular of the hiperprolactinemy.

#### **3.3.1.1 PK-PD model**

Mechanistic-model. Risperidone produces a blockade of D2 receptor in the tuberoinfundibular system causing a dose-dependent increase in prolactin that can be used for dosing it. Using the values of serum prolactin, obtained in the morning before administration of Risperidone and not before steady-state plasma levels , minimum concentration in "steady-state", are reached we can construct the dose-response curve of prolactin plasma level versus Risperidone dose, which fits a rectangular modified hyperbola, equation 5, and permit us to establish a maximum value of PRL= 80-90 (40-45) ng/ml and a minimum value of PRL= 40-50 (20-25) ng/ml, for the optimal dose of risperidone, in women and men, in parentheses (Lozano et al 2007, 2010c):

$$PRL = \frac{84Risp^2}{90 + Risp^2} \tag{5}$$

#### **3.3.1.2 PK-PG model**

Risperidone is eliminated by metabolism mainly through the action of citochrome CYP2D6, and to a lesser extent through CYP3A4. Risperidone and its main metabolite, 9-OHrisperidone, constitute the "active fraction" with an elimination half-life of 6 and 24 hours, respectively, in extensive metabolisers, EM, and 20 and 30 h in the case of patients with phenotype "poor metabolizer" PM. The "clearance", Cl, for the active fraction of Risperidone is respectively of 5.0 and 13.7 l/h in EM subjects and 3.2 and 3.3 l/h in individuals PM (Janssen Research Foundation 1994).

The CYP2D6 activity ranges considerably within a population and includes Ultrafast metabolizers, UM, Extensive metabolizers, EM, Intermediate metabolizers,IM, and Poor metabolizers, PM. Among these are fully functional alleles, alleles with reduced function and null, non-functional, alleles, which convey a wide range of enzyme activity, from no activity to ultrarapid metabolism of substrates. Null alleles of CYP2D6 do not encode a functional protein and have no detectable residual enzymatic activity, being responsibles for

Biomarkers and Therapeutic Drug Monitoring in Psychiatry 167

Mechanistic-model. Clozapine and Olanzapine, among other antipsychotics, cause an increase plasma cortisol levels in a dose-dependent manner, so the daily dosage has to be increased or decreased in order to reach cortisol plasma levels of 19.6 ± 6.6 mcg / dL and 18.3 ± 5.3 mcg / dL, the Clinical Surrogate Endpoint for Clozapine or Olanzapine,

Once the steady-state and a pharmacological response, ranging between the 20-80% of maximal effect, is reached, we can estimate the optimal dose required to achieve cortisol values of 19.6 ± 6.6 mcg / dL and 18.3 ± 5.3 mcg / dL, the Clinical Surrogate Endpoints for

Clozapine and Olanzapine are extensively metabolized in the liver, via the cytochrome P450 system, to polar metabolites suitable for its elimination in the urine and faeces. The CYP1A2 isoenzyme is primarily responsible for metabolism of Clozapine and Olanzapine , but another CYP`s seems to play a role, as well. Inducers agents, e.g. cigarette smoke, or inhibitor agents, e.g. Theophylline, Ciprofloxacin, Fluvoxamine, of the CYP1A2 may increase or decrease, respectively, the metabolism of Clozapine and Olanzapine. For example, the induction of metabolism caused by smoking means, that smokers would require double up the dose of Clozapine and/or Olanzapine compared with non-smokers ir

The poblational analysis of plasma cortisol levels, using Kernel`s test , allow us to detect 2 populations, sex-linked and related to an alterations of the dopaminergic and/or serotonergic, caused by a combination of the different polymorphisms of genes encoding SERT and 5-HT2a receptor, among others, obtaining from the application of the equation 2,

> DOSE genotype : wild type 2 or 4

The reduction of the testosterone levels induced by drugs with opioid activity, seems to be receptor mediated, since the different isomers have different activities: the levorotatory isomers are much more effective than dextrorotatory isomers (Cicero et al 1974, 1975, 1976,

There are many different studies showing that the relative potency of drugs to reduce serum testosterone levels is parallel to its analgesic activity and its affinity for opioid receptors in the brain. So, this ability to decrease plasma testosterone and/or urine can be used as a trial to evaluate the structure-activity relationship, kinetic constants of association-dissociation to receptors, and reach conclusions about their pharmacological potency and its optimal

DOSE genotype : polimorphic type

Clozapine or Olanzapine, respectively, using equation 2 (Lozano et al 2008a, 2011a).

order to achieve an equivalent plasma concentration (Entrez Gene 2011).

in dependence of the variant alleles present (Lozano et al 2008a, 2010c).

therapeutic dose (Cicero et al 1975, Mendelson et al 1976).

**3.3.2.1 PK-PD model** 

**3.3.2.2 PK-PD model** 

**3.3.2.3 PD-PG model** 

**3.4 Methadone** 

1977; Mendelson et al 1976).

the following equi-effective dose ratio:

respectively (Lozano et al 2007, 2008a, 2010c, 2011a).

the PM phenotype in homozygous and for IM phenotype in heterozygous, when to present (Zhou 2008), obtaining from the application of the equation 2, the following equi-effective dose ratios:

 DOSE CYP2D6 phenotype : EM 1.5 DOSE CYP2D6 phenotype : IM and DOSE CYP2D6 phenotype : EM <sup>3</sup> DOSE CYP2D6 phenotype : PM

(Lozano et al 2007, 2010c).

#### **3.3.1.3 PD-PG model**

The poblational analysis of PRL values, using Kernel`s test, allowed us to detect two subpopulations, sex-linked, and related to an alteration of the dopaminergic and / or serotonergic pathways, caused by a combination of polymorphisms of the genes encoding receptors 5-HT2a , D2, and SERT, among others, obtaining from the application of the equation 2, the following equi-effective dose ratio:

> DOSE genotype : wild type 0.4 / 2.5 DOSE genotype : polimorphic type

in men and women, respectivily (Lozano et al 2007, 2010c).

#### **3.3.2 Clozapine and olanzapine**

Since the HPA axis regulation appears altered in patients with schizophrenia, its modulation may be relevant for the control of symptoms in schizophrenia and antipsychotic treatment response. Otherwise, cortisol can be used as surrogate of the 5HT2a receptor blockade, the receptor that has manifested itself as the most important in the serotonergic regulation of HPA axis and, thus, can be used as Surrogate Marker for dosing antipsychotics that act predominantly on the serotonergic pathways (Marx & Lieberman 1998; Meltzer et al 2001; Morrow et al 1995). Moreover, the alteration of the HPA axis, induced by antipsychotic with capacity for serotoninergic antagonism, that bind strongly to receptors 5-HT2a/2c, can be used to modulate the response of cortisol and, eventually, to suppress the axis and to cause a decrease of the corticotropin-releasing factor, CRF, and the adrenocorticotropin hormone, ACTH (Morrow et al 1995; Patchev et al 1994).

Clozapine and Olanzapine, a 5HT2a antagonists, show strong affinity for some dopamine receptors, but weak ability to antagonize the D2 receptor, a receptor that modulates the neuroleptic activity. Evidence has shown that cortisol levels achieved in individuals treated with olanzapine and clozapine are dose dependent and directly proportional to the plasma concentrations of these drugs, at the start of antipsychotic treatment and along it.

Not only Olanzapine and Clozapine have shown a dose-dependent effect on the plasma cortisol levels, Risperidone also has significant effects but less pronounced compared with Olanzapine and Clozapine, whereas, Haloperidol has more modest effects. Olanzapine and Clozapine produce a cortisol increases, Δ-delta, up to 4-5 times higher than those produced by Haloperidol, while, risperidone produces a cortisol increases almost 1.5 times greater than observed with Haloperidol. Therefore, antipsychotics dosage are suitable to be monitored by using cortisol plasma levels (Girdler et al 2001).
