Clinical relevance of all-trans retinoic acid pharmacokinetics and its modulation in acute promyelocytic leukemia

Cardiac retinoic acid levels decline in heart failure

Although low circulating levels of the vitamin A metabolite, all-trans retinoic acid (ATRA), are associated with increased risk of cardiovascular events and all-cause mortality, few studies have addressed whether cardiac retinoid levels are altered in the failing heart. Here, we showed that proteomic analyses of human and guinea pig heart failure (HF) were consistent with a decline in resident cardiac ATRA. Quantitation of the retinoids in ventricular myocardium by mass spectrometry revealed 32% and 39% ATRA decreases in guinea pig HF and in patients with idiopathic dilated cardiomyopathy (IDCM), respectively, despite ample reserves of cardiac vitamin A. ATRA (2 mg/kg/d) was sufficient to mitigate cardiac remodeling and prevent functional decline in guinea pig HF. Although cardiac ATRA declined in guinea pig HF and human IDCM, levels of certain retinoid metabolic enzymes diverged. Specifically, high expression of the ATRA-catabolizing enzyme, CYP26A1, in human IDCM could dampen prospects for an ATRA-based therapy. Pertinently, a pan-CYP26 inhibitor, talarozole, blunted the impact of phenylephrine on ATRA decline and hypertrophy in neonatal rat ventricular myocytes. Taken together, we submit that low cardiac ATRA attenuates the expression of critical ATRA-dependent gene programs in HF and that strategies to normalize ATRA metabolism, like CYP26 inhibition, may have therapeutic potential.

Therapeutic potential of the inhibition of the retinoic acid hydroxylases CYP26A1 and CYP26B1 by xenobiotics

Retinoic acid (RA), the active metabolite of vitamin A, is an important endogenous signaling molecule regulating cell cycle and maintenance of epithelia. RA isomers are also used as drugs to treat various cancers and dermatological diseases. However, the therapeutic uses of RA isomers are limited due to side effects such as teratogenicity and resistance to treatment emerging mainly from autoinduction of RA metabolism. To improve the therapeutic usefulness of retinoids, RA metabolism blocking agents (RAMBAs) have been developed. These inhibitors generally target the cytochrome P450 (CYP) enzymes because RA clearance is predominantly mediated by P450s. Since the initial identification of inhibitors of RA metabolism, CYP26 enzymes have been characterized as the main enzymes responsible for RA clearance. This makes CYP26 enzymes an attractive target for the development of novel therapeutics for cancer and dermatological conditions. The basic principle of development of CYP26 inhibitors is that endogenous RA concentrations will be increased in the presence of a CYP26 inhibitor, thus, potentiating the activity of endogenous RA in a cell-type specific manner. This will reduce side effects compared to administration of RA and allow for more targeted therapy. In clinical trials, inhibitors of RA metabolism have been effective in treatment of psoriasis and other dermatological conditions as well as in some cancers. However, no CYP26 inhibitor has yet been approved for clinical use. This review summarizes the history of development of RAMBAs, the clinical and preclinical studies with the various structural series and the available knowledge of structure activity relationships of CYP26 inhibitors.

The role of CYP26 enzymes in retinoic acid clearance

Retinoic acid (RA) is a critical signaling molecule that regulates gene transcription and the cell cycle. Understanding of RA signaling has increased dramatically over the past decades, but the connection between whole body RA homeostasis and gene regulation in individual cells is still unclear. It has been proposed that cytochrome P450 family 26 (CYP26) enzymes have a role in determining the cellular exposure to RA by inactivating RA in cells that do not need RA. The CYP26 enzymes have been shown to metabolize RA efficiently and they are also inducible by RA in selected systems. However, their expression patterns in different cell types and a mechanistic understanding of their function is still lacking. Based on preliminary kinetic data and protein expression levels, one may predict that if CYP26A1 is expressed in the liver at even very low levels, it will be the major RA hydroxylase in this tissue. As such, it is an attractive pharmacological target for drug development when one aims to increase circulating or cellular RA concentrations. To further the understanding of how CYP26 enzymes contribute to the regulation of RA homeostasis, structural information of the CYP26s, commercially available recombinant enzymes and good specific and sensitive antibodies are needed.

Apoptotic events induced by naturally occurring retinoids ATRA and 13-cis retinoic acid on human hepatoma cell lines Hep3B and HepG2

Two hepatoma cell lines were incubated for 72 h with ATRA and its analog 13cisRA and according to MTT assay, Hep3B cells were highly susceptible whereas HepG2 cells were more resistant to the treatment. At the high concentration of 166 microM, retinoids were able to induce apoptosis in both cell lines and the highest effect was observed in HepG2 cells treated with ATRA. TUNEL-based photometric ELISA showed that at the same retinoid concentration tested by flow cytometry, both cell lines showed apoptosis whereas plasma membranes were not significantly disrupted. Inhibitors of apoptosis Bcl-xL and survivin were downregulated in Hep3B cells by treatment with both retinoids. Bax, a pro-apoptotic protein, was not significantly upregulated in Hep3B cells, but was slightly increased in HepG2 cells treated with 13cisRA. Both procaspase-3 and procaspase-8 were cleaved in Hep3B cells, suggesting apoptosis could be triggered through the extrinsic pathway. In the case of HepG2 cells, lack of caspase activation suggests a mechanism dependent on other kind of proteases.

Pharmacokinetic study of all-trans-retinoyl-beta-D-glucuronide in Sprague-Dawley rats after single and multiple intravenous administration(s)

All-trans-retinoyl-beta-D-glucuronide (RAG) is an endogenous active metabolite of all-trans-retinoic acid (ATRA). In the present study, the pharmacokinetics of RAG was examined after the administration of a single intravenous does (5, 10, or 15 micromol/kg) and of multiple daily intravenous doses (5 micromol/kg) to rats for 8 days. The plasma concentrations of RAG and ATRA were measured by a reverse-phase HPLC method. A rapid distribution phase of approximately 1 h was observed in all of the rats after single or multiple doses. Thereafter, RAG was eliminated through a first-order process, in accord with a typical two-compartment first order pharmacokinetic profile. After single intravenous doses, the AUC of RAG increased proportionally with the dose and the clearance remained unchanged within the tested doses. There was no statistical significant difference in distribution rate constants from central compartment to peripheral compartment (K(12)) and from peripheral compartment to central compartment (K(21)) between different doses. However, as the dose increased from 5 micromol/kg to 10 micromol/kg, the volume of distribution at the steady state (V(ss)) and the volume of peripheral compartment (V(p)) decreased significantly (p < 0.05) from 1.290 +/- 0.269, 0.928 +/- 0.232. L/kg to 0.961 +/- 0.149, 0.647 +/- 0.107 L/kg, respectively. V(ss) and V(p) at a dose of 15 micromol/kg (0.924 +/- 0.187, 0.698 +/- 0.165 L/kg) were not significantly different from that at 10 micromol/kg. Thus, RAG might saturate the tissue-binding sites at higher doses. ATRA was detected as a metabolite of RAG at low levels (usually < 0.05 microM) only in the first 2 h after intravenous administration. RAG clearly was not extensively hydrolyzed to ATRA in our study. After multiple daily intravenous administration of RAG, the clearance (Cl) and the elimination rate constant (K(10)) remained unchanged (p > 0.05), indicating that long-term daily administration of RAG did not induce its accelerated metabolism. However, K(12), V(p), and V(ss) declined significantly (p < 0.05) from 1.67 +/- 0.54 h(-1), 0.928 +/- 0.232 L/kg, and 1.290 +/- 0.269 L/kg to 0.96 +/- 0.48 h(-1), 0.494 +/- 0.147 L/kg, and 0.818 +/- 0.187 L/kg, respectively. Therefore, long-term daily dosing of RAG seemed to decrease its distribution profile. Although the AUC of RAG did not change significantly after multiple dosing, the AUC of ATRA after RAG dosing significantly declined (p < 0.05) from 0.032 +/- 0.019 microM x h to 0.010 +/- 0.006 microM x h. The decline in the AUC of ATRA might reflect an increase in its uptake by tissue and/or in its metabolism. Because enhanced clearance is not associated with RAG after multiple administrations, RAG could be considered as an alternate to ATRA in appropriate clinical applications.

Treatment of newly diagnosed and relapsed acute promyelocytic leukemia with intravenous liposomal all-trans retinoic acid

A novel intravenous liposomal formulation of all-trans retinoic acid (ATRA) was evaluated in 69 patients with acute promyelocytic leukemia (APL): 32 new diagnoses, 35 relapses, and 2 oral ATRA failures. Liposomal ATRA (90 mg/m(2)) was administered every other day until complete remission (CR) or a maximum of 56 days. Treatment following CR was liposomal ATRA with or without chemotherapy. In an intent-to-treat (ITT) analysis of all patients, CR rates were 62%, 70%, and 20% in newly diagnosed, group 1 first relapses (ATRA naive or off oral ATRA more than or equal to 1 year), or group 2 relapses (second or subsequent relapse or first relapses off oral ATRA less than 1 year), respectively. In 56 evaluable patients (receiving 4 or more doses), CR rates for the same groups were 87% (20 of 23), 78% (14 of 18), and 23% (3 of 13). Remission failure in newly diagnosed patients was not from resistant disease. Several patients in CR became polymerase chain reaction (PCR) negative for promyelocytic leukemia/retinoic acid receptor-alpha (PML/RARalpha) after liposomal ATRA alone. Toxicity was generally mild, most commonly headaches (67. 5%). Eighteen patients (26%) had ATRA syndrome develop during induction. One-year survival of ITT patients was 62%, 56%, and 20% for newly diagnosed, group 1, and group 2, respectively. The medium duration of CR has not yet been reached and was 18 and 5.5 months in the same groups. These results demonstrate that liposomal ATRA is effective in inducing CR in newly diagnosed or group 1 APL patients. It provides a reliable dosage of ATRA for patients with APL unable to swallow or absorb medications and can induce molecular remissions without chemotherapy.

Identification of human cytochrome P450s involved in the formation of all-trans-retinoic acid principal metabolites

Cytochrome P450 (P450)-dependent metabolism of all-trans-retinoic acid (atRA) is important for the expression of its biological activity. Because the human P450s involved in the formation of the principal atRA metabolites have been only partially identified, the purpose of this study was to identify the human P450s involved in atRA metabolism. The use of phenotyped human liver microsomes (n = 16) allowed the identification of the following P450s: 2B6, 2C8, 3A4/5, and 2A6 were involved in the formation of 4-OH-RA and 4-oxo-RA; 2B6, 2C8, and 2A6 correlated with the formation of 18-OH-RA; and 2A6, 2B6, and 3A4/5 activities correlated with 5, 6-epoxy-RA formation (30-min incubation, 10 microM atRA, HPLC separation, UV detection 340 nm). The use of 15 cDNA-expressed human P450s from lymphoblast microsomes, showed the formation of 4-OH-RA by CYP3A7 > CYP3A5 > CYP2C18 > CYP2C8 > CYP3A4 > CYP2C9, whereas the 18-OH-RA formation involved CYPs 4A11 > 3A7 > 1A1 > 2C9 > 2C8 > 3A5 > 3A4 >2C18. Kinetic studies identified 3A7 as the most active P450 in the formation of three of the metabolites: for 4-OH-retinoic acid, 3A7 showed a V(max)/K(m) of 127.7, followed by 3A5 (V(max)/K(m) = 25.6), 2C8 (V(max)/K(m) = 24.5), 2C18 (V(max)/K(m) = 15.8), 3A4 (V(max)/K(m) = 5.7), 1A1 (V(max)/K(m) = 5.0), and 4A11 (V(max)/K(m) = 1.9); for 4-oxo-RA, 3A7 showed a V(max)/K(m) of 13.4, followed by a 10-fold lower activity for both 2C18 and 4A11 (V(max)/K(m) = 1.2); and for 18-OH-RA, 3A7 showed a V(max)/K(m) of 10.5 compared with a V(max)/K(m) of 2.1 for 4A11 and 2.0 for 2C8. 5,6-Epoxy-RA was only detected at high substrate concentrations in this system (>10 microM), and P450s 2C8, 2C9, and 1A1 were the most active in its formation. The use of embryonic kidney cells (293) stably transfected with human P450 cDNA confirmed the major involvement of P450s 3A7, 1A1, and 2C8 in the oxidation of atRA, and to a lesser extent, 1A2, 2C9, and 3A4. In conclusion, several human P450s involved in atRA metabolism have been identified, the expression of which was shown to direct atRA metabolism toward the formation of specific metabolites. The role of these human P450s in the biological and anticancer effects of atRA remains to be elucidated.

Kinetic study of a 2-hydroxypropyl-beta-cyclodextrin-based formulation of all-trans-retinoic acid in Sprague-Dawley rats after oral or intravenous administration

all-trans-Retinoic acid (ATRA, vitamin A acid, or tretinoin) is a potent chemotherapeutic agent for the treatment of acute promyelocytic leukemia (APL). Its poor aqueous solubility not only affects its oral absorption but also prevents it from forming an aqueous parenteral formulation. Recently, we developed a water-soluble formulation of ATRA with 2-hydroxypropyl-beta-cyclodextrin (HPbetaCD). In present study, this formulation was tested in Sprague-Dawley rats. Kinetic study of ATRA was carried out after oral or intravenous administration. Though there were no statistical differences in any of the estimated pharmacokinetic parameters between ATRA sodium salt and HPbetaCD-based ATRA after intravenous administration, inclusion of ATRA into HPbetaCD was found to greatly improve the oral absorption of ATRA.

Assessment of all-trans retinoic acid (ATRA) efficacy as a single agent in primary lymphoid neoplasia

All-trans retinoic acid (ATRA) is currently widely used in the therapy of acute promyelocytic leukemia and is being tested in vitro and in vivo on several other malignancies. Previously ATRA has been shown to inhibit the growth in vitro, of established human myeloma cell lines as well as cultured primary myeloma cells from patients. ATRA acts by down-regulating IL-6-receptor-alpha or gp130 on the surface of the myeloma cells. However, despite its in vitro effects on myeloma cells, ATRA therapy on advanced stage multiple myeloma (MM) patients has so far largely been ineffective. In current studies, we have assessed the efficacy of ATRA therapy against primary murine plasma cell tumors, which are an animal model for human MM. These tumors are induced at about 50% incidence in pristane-primed BALB/c mice by injection of v-raf/v-myc- containing retroviruses and are IL-6 dependent. Using this animal model, we assessed the effect of ATRA as a therapeutic agent against primary tumors at two early time points in disease development. ATRA was administered in liposomal vesicles (ATRAGEN), since liposomal-ATRA has been shown to circumvent clearance mechanisms by hepatic microsomes, which normally occur with free ATRA. In addition, ATRAGEN was previously shown to be less toxic in mice than free ATRA. ATRAGEN was administered beginning on day 25 or day 45 after virus injection and continued twice weekly for 8-11 weeks. ATRAGEN administration begun at either time point did not alter the incidence or the latency of plasma cell tumors compared with control animals. These results suggest that ATRA may not be an effective sole therapy against early MM.

Differences in the lipoprotein distribution of free and liposome-associated all-trans-retinoic acid in human, dog, and rat plasma are due to variations in lipoprotein lipid and protein content

The objective of the proposed study was to determine the distribution in plasma lipoprotein of free all-trans retinoic acid (ATRA) and liposomal ATRA (Atragen; composed of dimyristoyl phosphatidylcholine and soybean oil) following incubation in human, rat, and dog plasma. When ATRA and Atragen at concentrations of 1, 5, 10, and 25 micrograms/ml were incubated in human and rat plasma for 5, 60, and 180 min, the majority of the tretinoin was recovered in the lipoprotein-deficient plasma fraction. However, when ATRA and Afragen were incubated in dog plasma, the majority of the tretinoin (> 40%) was recovered in the high-density lipoprotein (HDL) fraction. No differences in the plasma distribution between ATRA and Atragen were found. These data suggest that a significant percentage of tretinoin associates with plasma lipoproteins (primarily the HDL fraction) upon incubation in human, dog, and rat plasma. Differences between the lipoprotein lipid and protein profiles in human plasma and in dog and rat plasma influenced the plasma distribution of ATRA and Atragen. Differences in lipoprotein distribution between ATRA and Atragen were not observed, suggesting that the drug's distribution in plasma in not influenced by its incorporation into these liposomes.

Acute promyelocytic leukemia as a model for cross-talk between interferon and retinoic acid pathways: from molecular biology to clinical applications

Acute promyelocytic leukemia (APL) has been regarded as the paradigm for therapeutic approaches utilizing differentiating agents, due to the fact that almost 95% of patients undergo complete remission when treated with all-trans retinoic acid (ATRA). However, complete clinical remission with ATRA alone is always transient, and relapse in APL is almost invariably associated with the acquisition of resistance to ATRA. Acquired resistance to ATRA in APL cell lines and in some APL clinical cases can be partially overcome by interferons (IFNs), cytokines which have well established tumor-growth suppressive activities. APL is associated in 99% of cases with a 15;17 translocation that fuses the PML and Retinoic Acid Receptor alpha (RARalpha) genes. RARalpha is one of the Retinoic Acid (RA) nuclear receptors which mediates, at the transcriptional level, ATRA differentiating and growth suppressive activity. PML is a tumor-growth suppressor whose expression is directly regulated by IFNs. Here we review the molecular mechanisms by which IFNs and RA can cooperate in controlling cell growth and differentiation of normal hemopoietic cells and leukemic cells, focusing on APL as a model system.