Contents
- Introduction 5
- Chemistry 7
- Pharmacological aspects 8
- Pharmacokinetics 8
- Absorption 8
- Distribution 9
- Metabolism 11
- Elimination 14
- Pharmacodynamics and pharmacological effects 15
- Toxicity 20
- New perspectives for mitotane treatment 22
- Conclusions and future directions 25
Introduction
Adrenocortical carcinoma (ACC) is a rare endocrine malignancy arising
from one of the three cortical layers of the adrenal gland. ACCs can
cause an increase in the production of one or more steroid hormones,
such as cortisol, androgens, and aldosterone, resulting in clinical
manifestations of Cushing’s syndrome, virilization (facial acne, pubic
pilification, hirsutism, increase in muscle mass), and high blood
pressure, respectively. Clinical manifestations are variable, and the
prognosis of ACC is often unfavorable, especially at older ages
[1,2]. The general incidence of ACC is 1.7–2.0 cases/million/year
among adults, whereas it is even rarer among children in most countries
(0.2-0.3 cases/million/year); it is more common among women [2]. One
exception is Southern Brazil, wherein the ACC incidence is higher in
children than in adults [3], reaching a rate of 3.4–4.2 per million
children versus an estimated worldwide incidence of 0.3 per one million
children younger than 15 years [4].
Currently, mitotane is the only approved drug, as single or adjuvant
treatment, for post-operative or inoperable ACC. The daily dose usually
ranges between 2 and up to 10 g to reach the ideal plasma concentration
with tolerable toxicity, which is 14–20 mg L-1[5,6]. The mean dose is 4 g day-1 for adults and 4
g m²/day-1 for children for ACC stage 3/4 and
recurrence of stage 1/2 [7,8]. In most patients, the steady state is
reached after 3 months of daily treatment [6,9]. Nevertheless,
patients experienced several adverse events of the gastrointestinal
(diarrhea, nausea, vomiting, and anorexia) and central nervous
(confusion, ataxia, and dizziness) systems [10] and many other
adverse effects that were not well documented (ClinicalTrials.gov
Identifier: NCT00568139), limiting its therapeutic use mainly among
pediatric ACC patients.
Mitotane [1-(o-chlonophenyl)-1-(p-chIorophenyl) 2,2-dichioroethane],
also known as o,p′-DDD, was isolated in 1940 from the insecticide
dichlorodiphenyltrichloroethane (DDT). The adrenolytic effect of
mitotane was first reported in dogs for inducing selective necrosis in
the zona fasciculata and zona reticularis of the adrenal
cortex [11,12]. The first clinical evidence was published in 1959,
when the efficacy of mitotane for the treatment of ACC was reported in
male patients [13]. Thereafter, even though the mechanism of action
was unclear, mitotane was rapidly approved by the Food and Drug
Administration (FDA) for ACC treatment, and it became commercially
available in a tablet form as the reference product
Lysodren® [14].
The first line treatment for ACC after surgery is mitotane monotherapy
for less progressive ACC or mitotane plus chemotherapy, such as
etoposide, doxorubicin, and cisplatin (mitotane + EDC), to treat more
aggressive forms [15,16]. However, when first line therapy fails,
mitotane can also be employed in combination with streptozotocin,
vincristine, gemcitabine + capecitabine, and avelumab [15]. A recent
meta-analysis revealed that mitotane monotherapy with or without
radiotherapy, decreased the recurrence rate and mortality after tumor
resection among patients without distant metastasis [17]. However,
the efficacy of mitotane combined with chemotherapy is still unclear. To
evaluate if adjuvant mitotane + EDC, after resection of ACC, has
survival benefits for patients, we searched for “mitotane AND survival
AND adrenocortical carcinoma OR adrenal tumor” in PubMed, Cochrane, Web
of Science, and Scopus between November and December of 2019. We
identified 851 records in the databases. Of these, 298 were duplicated
and 553 were screened by title and abstract evaluation. Thirty-one
full-text articles were assessed for eligibility. None of these articles
met the inclusion criteria for meta-analysis of the benefits of adjuvant
mitotane plus chemotherapy in prolonging the survival of patients. Thus,
the clinical efficacy of mitotane in prolonging patient survival is
still controversial due to a lack of randomized controlled studies.
However, some clinical trials of mitotane + EDC for advanced ACC,
recently registered in the Cochrane library, may provide useful data for
the evaluation of patient survival in the future. Among the seven trials
evaluating mitotane therapy plus chemotherapy identified in our study,
only two have monitored the plasma mitotane levels [18,19] (Table
1).
Even after five decades of the mitotane approval by FDA, few studies
have addressed pharmacological aspects of mitotane, probably because ACC
is a rare disease. Therefore, better understanding of the effects of
mitotane on ACC is needed to improve the clinical monitoring of its
efficacy and development of new drugs with specific action against ACC
with fewer toxic effects. Thus, this review summarized the chemistry,
pharmacokinetic and pharmacodynamic features, and therapeutic and toxic
effects of mitotane and the new formulations containing this drug.
Chemistry
Unlike DDT, the mitotane molecule exists in two configurations depending
on the position of the chlorine element in the benzene ring: the
position ortho in one ring and the position para in
another ring. These mitotane configurations are characterized by the
presence of an asymmetric chiral carbon atom that gives rise to two
enantiomeric (R and S ) molecules. Considering that
mitotane is available as a racemic mixture, both enantiomers,
(S)-(-)-o,p′-DDD and (R)-(+)-o,p′-DDD (Fig. 1A) are present in the
commercial drug. Furthermore, it has been demonstrated that mitotane
metabolism involves two reactions via α- and β-hydroxylation.
α-hydroxylation forms the metabolite o,p’-DDE (DDE), whereas
β-hydroxylation forms o,p’-dichlorodiphenyl acyl chloride (DDAC). DDAC
has a strong affinity for biological nucleophiles, which can acylate
different cellular molecules; otherwise, DDAC could be rapidly converted
to the metabolite o,p′-DDA (DDA) in the presence of water (Fig. 1B).
- Pharmacological aspects
- Pharmacokinetics
3.1.1. Absorption
Mitotane has poor water solubility (0.1 mg mL-1 at
25ºC), being better soluble in organic solvents such as ethanol (20 mg
mL-1), dimethylsulfoxide (DMSO) (30 mg
mL-1), and dimethyl formamide (DMF) (PubChem; Cayman
Chemical). In pre-clinical studies, mitotane was often dissolved in
olive oil or DMSO followed by aqueous buffer, to reach ideal plasma
concentrations [24,25]. In clinical tests, mitotane reached the
maximum absorption when administered together with dietary lipids such
as high fat milk, chocolate, or oil emulsions, which increased its
absorption by 5-fold compared to that of mitotane tablets [26].
Although mitotane (single dose of 2 g) reached the maximal plasma
concentration of 0.0016 mg mL-1 (data from 9 patients) 10 h after
administration [26], the time to reach the ideal plasma
concentration (14 mg L-1) among ACC patients was around 116 days with an
accumulative dose of approximately 626 g (data from 53 patients)
[27]. However, it is difficult to determine mitotane bioavailability
since there is no data about in vitro intestinal absorption,
bioavailability of intravenously administered mitotane, or hepatic
extraction to determine whether mitotane has first-pass metabolism.
Despite the lack of information about the pharmacokinetics of mitotane,
there is no doubt that the oral bioavailability is low, probably due to
poor absorption and extensive metabolism. Indeed, patients required high
doses (2–10 g day-1) to reach the therapeutic plasma
concentration (14-20 mg L-1) and the steady state was
reached only about 3 months after treatment initiation [5,6]. In
addition, high variation in plasma concentration was observed with
different doses among patients probably due to the basic individual
variability in body mass index, sex, age, and metabolic enzyme activity.
Metabolic cytochrome P450 (CYP) polymorphisms might play an important
role in mitotane absorption. In fact, D’Avolio et al. (2013) [27]
demonstrated that the polymorphism of CYP2B6 was positively correlated
with higher plasma mitotane concentration. In patients with any stage
ACC treated with mitotane monotherapy, the polymorphism of CYP2W1 was
associated with a reduced probability of reaching the mitotane
therapeutic range and with lower response rates, whereas the
polymorphism of CYP2B6 was correlated with high plasma levels of
mitotane [28]. It was documented that mitotane is a CYP2B inducer
(see section 3.1.3); however, the roles of CYP2B6 and CYP2W1 in mitotane
functions are not yet clear. Thus, Kerkhofs and colleagues (2015)
[29] proposed a new pharmacokinetic model adapted to individual
patients in daily clinical practice. The 3-compartment model was
proposed to measure mitotane levels through the volumes of distribution
and clearance [29]. Therefore, this model can be used as a tool to
elucidate the absorption of mitotane and the variability among patients.
3.1.2. Distribution
The distribution of mitotane in various body compartments has been
studied for a long time. The estimated volume of distribution was
relatively high. Thus, the plasma concentration was more influenced by
mitotane distribution than the elimination process [29]. The maximum
concentration of mitotane can be detected in the plasma of patients 2–8
hours after a single dose of 2 g [26]. Nevertheless, DDA levels were
higher than mitotane levels in the plasma, whereas DDE was barely
detected after the administration of repeated doses [30]. However,
the mitotane concentration in adipose tissue was 200-fold higher than
the plasma concentration in patients during chemotherapy [31]. In
rats, after 80 days of diet containing 1% mitotane, mitotane tended to
accumulate mainly in adipose tissue, as well as in the liver, kidney,
brain, and adrenal tissue [32]. Mitotane labeled with14C was localized mainly in the zona
fasciculata and zona reticularis in human adrenocortical tumor
and mouse adrenal tissue [33,34].
Distribution of mitotane involved chylomicron and lipoprotein binding as
a higher proportion of plasma mitotane was bound to lipoproteins
[35]. Under normolipidemic conditions, a substantial amount of
mitotane was bound to high density lipoprotein and albumin, whereas
under hypertriglyceridemic conditions, mitotane was bound mainly to
chylomicron and to very low-density lipoproteins (VLDL) [36]. The
distribution of mitotane involves cellular diffusion by adhering to LDL.
Interestingly, mitotane promoted LDL formation [37], and probably
promoted its self-uptake into adrenals cells. However, in vitro ,
lipoprotein binding inhibited the activity of mitotane, suggesting that
lipoprotein-free mitotane was its therapeutically active fraction
[35,38]. Higher antiproliferative and proapoptotic effects of
mitotane were shown in H295R cells grown in lipoprotein-free medium, and
a higher rate of tumor control was demonstrated in patients with ACC
treated with mitotane and statins [38]. Under hypertriglyceridemic
conditions, the bound mitotane-VLDL complex does not enter into the
adrenal cells [36], and the
patient may be unresponsive to the effects of mitotane. High
concentrations of DDA and DDE were also found bound to chylomicrons.
However, it is important to note that DDA is not too lipophilic due to
the presence of the carboxylic acid group, whereas mitotane and DDE have
a high affinity for lipids [35]. Therefore, to estimate toxicity and
efficacy and improve patient care, it is important to quantify
lipoprotein-free and lipoprotein-bound mitotane [35].
Regarding the enantiomers of mitotane, different elimination ratios were
suggested in the exposure of enantiomers to human placentas as measured
by gas chromatography [39]. S(-)-o,p′-DDD was dominant in the plasma
of two minipigs whereas the enantiomer R-(+)-o,p′-DDD was dominant in
the plasma of three minipigs [40]. Besides, the two isomers were
equally potent in decreasing H295R cell viability, (S)-(-)-o,p′-DDD
affected hormone secretion (dehydroepiandrosterone [DHEA] and
cortisol) slightly less than the (R)-(+)-o,p′-DDD and the racemic
mixture [41]. Thus, it is also suggested that the racemic mixture is
important for the therapeutic effects of mitotane. However, the
pharmacological features of each enantiomer in ACC treatment are not yet
clear.
3.1.3. Metabolism
The metabolism of mitotane was previously described as occurring only in
adrenal cells [42]. This drug has two reactions in the mitochondria
of adrenal cells: α-hydroxylation which yields the end-product DDE, and
β-hydroxylation which gives rise to the end-product DDA (Fig. 1B).
Although measurements of plasma DDA for estimating the effects of
mitotane suggested that it was an active metabolite [30], other
authors claimed that DDA was an inactive metabolite [38]. In fact,
DDA is not so lipophilic and its presence is further found in the urine
(see section 3.1.4), suggesting that DDA is an end-product of the
mitotane deactivation pathway. In contrast, it has been speculated that
DDE could be an active metabolite, since it was not extensively found in
plasma, urine, or feces (see section 3.1.4) and had a cytotoxic effect
on the H295R adrenocortical cell line [43]. However, no additional
studies have been conducted yet to confirm this hypothesis.
The β-hydroxylation of mitotane has another important function beyond
DDA formation, which is the generation of an intermediate reactive
metabolite, an acyl chloride derivative (DDAC), which might be
responsible for covalent binding to mitochondrial macromolecules in the
adrenals [44]. It is important to point out that the covalent bond
is not permanent and can be reversed by the addition of glutathione
reductase (GSH), suggesting that GSH can inactivate mitotane [45].
The reaction for the covalent bond is mediated through a specific CYP450
responsible for steroidogenesis in the adrenal cells, such as CYP11A1
(see section 3.2) [46]. In the presence of a P450 inhibitor
(erythromycin), mitotane and DDA tend to accumulate in fat tissue and
plasma, whereas the levels of DDE decrease [32], supporting the
theory that P450 is responsible for α-hydroxylation of mitotane. In
addition, accumulation of 14C labeled mitotane in the
adrenals was partially reversed by metyrapone, a known inhibitor of
CYP11B1 [33,34], suggesting that both CYP11A1 and CYP11B1 are
involved in the formation of the DDAC.
Other CYPs are also related to the effects of mitotane. Recently, Murtha
and colleagues demonstrated that silencing of CYP2A6 mRNA in H295R cells
promoted higher sensitivity of this cell line to the effects of mitotane
[47]. This enzyme is a known metabolizer of some xenobiotics;
however, the exact function of CYP2A6 in ACC is still unclear. Moreover,
high expression level of CYP2W1 mRNA was observed in both normal and
neoplastic adrenal glands and was related to a better response to
mitotane treatment among patients [48]. It was suggested that CYP2W1
might play a role in mitotane metabolism and could be a potential
predictive marker of sensitivity to mitotane treatment [49];
however, the tissue expression and function of CYP2W1 are still under
debate.
When mitotane was administered to mice, via the intraperitoneal route
(440 mg kg-1, dissolved in olive oil, for 48 days),
the antitumor effects remained for 34 days, compared to those achieved
by oral administration, which yielded antitumor effects for only 13 days
[25]. This suggests that mitotane undergoes first pass metabolism
when administered orally and/or is largely eliminated via feces.
Furthermore, the intestinal microbiome could also influence mitotane
absorption and metabolism. The hypothesis of intestinal mitotane
metabolism is supported by high concentrations of DDA and DDE found in
chyle. Moreover, DDE might be metabolized further in the liver, which
would explain why this compound was barely detected in the plasma
[35]. This indicates that mitotane might be metabolized in the liver
via enzymatic (i.e. via CYP450) and non-enzymatic (i. e., glutathione
[GSH]) reactions since DDT is metabolized in the liver in the same
way [50]. However, no preclinical studies have investigated the
metabolism of mitotane, DDA, and DDE in the liver. Moreover, the role of
the intestinal microbiome in mitotane metabolism should also be
considered, since the gut microbiota plays a role in drug metabolism
prior to absorption or during enterohepatic circulation, via various
microbial enzymatic reactions in the intestine [51]. Therefore,
understanding the effects of liver and gut microbiota on mitotane
metabolism is crucial to explaining the changes in its pharmacokinetics.
Mitotane has a narrow therapeutic index; thus, the possibility of
pharmacokinetic drug–drug interactions should not be discarded. This
drug has a long-lasting inducing effect on CYP3A4 and CYP2B and a potent
inhibitory effect on CYP2C19, which result in clinical interactions with
many drugs metabolized by these enzymes [52,53,54]. Therefore, when
associative treatment of mitotane in combination with such drugs (i.e.
doxorubicin, etoposide, hydrocortisone, cyclophosphamide, omeprazole,
and clopidogrel) is necessary, the therapeutic effects and toxicity of
these drugs need to be monitored closely.
3.1.4. Elimination
After an oral intake of a single mitotane dose in tablet form (2 g),
around 40% of unchanged mitotane could be detected in the feces after
12 h, whereas mitotane in milk or emulsion decreased the amount of drug
excreted to less than 10% [26]. Furthermore, after oral mitotane
treatment (100 mg) in rats, 7.1% was excreted in the urine and 87.8%
of mitotane was found in the feces within 8 days [55]. Thus,
unchanged mitotane appears to be eliminated largely through biliary
excretion. However, mitotane, DDA, and DDE can also undergo
enterohepatic circulation; chyle could be enriched due to hepatic
transformation [35]. The mitotane clearance and volume of
distribution in the steady state were determined in 22 patients as 0.94
± 0.37 L h-1 and 161 ± 68 L kg-1 of
the lean body mass, respectively [29]. In addition, DDA was found in
the urine of mice treated with mitotane (250 mg kg-1,
p.o.) for a total of 96 h [24] as well as in the feces [56].
Although renal elimination also plays an important role in mitotane
clearance, the rate is lower than that of biliary elimination. In a
study of 19 patients receiving mitotane at 3–6 g
day-1 over a period of 30–60 days, the half-life was
found to be between 18 and 159 days after mitotane was withdrawn
[26]. Long-term mitotane elimination observed was probably due to
its accumulation in fat tissue, attributable to its lipophilic
characteristics.