Tretinoin. A review of its pharmacodynamic and pharmacokinetic properties and use in the management of acute promyelocytic leukaemia

Targeting breast cancer stem cells through retinoids: A new hope for treatment

Breast cancer is a complex and diverse disease accounting for nearly 30% of all cancers diagnosed in females. But unfortunately, patients develop resistance to the existing chemotherapeutic regimen, resulting in approximately 90% treatment failure. With over half a million deaths annually, it is imperative to explore new therapeutic approaches to combat the disease. Within a breast tumor, a small sub-population of heterogeneous cells, with a unique ability of self-renew and differentiation and responsible for tumor formation, initiation, and recurrence are referred to as breast cancer stem cells (BCSCs). These BCSCs have been identified as one of the main contributors to chemoresistance in breast cancer, making them an attractive target for developing novel therapeutic strategies. These cells exhibit surface biomarkers such as CD44+, CD24-/LOW, ALDH, CD133, and CD49f phenotypes. Higher expression of CD44+ and ALDH activity has been associated with the formation of tumors in various cancers. Moreover, the abnormal regulation of signaling pathways, including Hedgehog, Notch, β-catenin, JAK/STAT, and P13K/AKT/mTOR, leads to the formation of cancer stem cells, resulting in the development of tumors. The growing drug resistance in BC is a significant challenge, highlighting the need for new therapeutic strategies to combat this dreadful disease. Retinoids, a large group of synthetic derivatives of vitamin A, have been studied as chemopreventive agents in clinical trials and have been shown to regulate various crucial biological functions including vision, development, inflammation, and metabolism. On a cellular level, the retinoid activity has been well characterized and translated and is known to induce differentiation and apoptosis, which play important roles in the outcome of the transformation of tissues into malignant. Retinoids have been investigated extensively for their use in the treatment and prevention of cancer due to their high receptor-binding affinity to directly modulate gene expression programs. Therefore, in this study, we aim to summarize the current understanding of BCSCs, their biomarkers, and the associated signaling pathways. Retinoids, such as Adapalene, a third-generation retinoid, have shown promising anti-cancer potential and may serve as therapeutic agents to target BCSCs.

Characterization of the SARS-CoV-2 E Protein: Sequence, Structure, Viroporin, and Inhibitors

The COVID-19 epidemic is one of the most influential epidemics in history. Understanding the impact of coronaviruses (CoVs) on host cells is very important for disease treatment. The SARS-CoV-2 envelope (E) protein is a small structural protein involved in many aspects of the viral life cycle. The E protein promotes the packaging and reproduction of the virus, and deletion of this protein weakens or even abolishes the virulence. This review aims to establish new knowledge by combining recent advances in the study of the SARS-CoV-2 E protein and by comparing it with the SARS-CoV E protein. The E protein amino acid sequence, structure, self-assembly characteristics, viroporin mechanisms and inhibitors are summarized and analyzed herein. Although the mechanisms of the SARS-CoV-2 and SARS-CoV E proteins are similar in many respects, specific studies on the SARS-CoV-2 E protein, for both monomers and oligomers, are still lacking. A comprehensive understanding of this protein should prompt further studies on the design and characterization of effective targeted therapeutic measures.

Anti-neoplastic agents for patients on peritoneal dialysis: A systematic review

BACKGROUND

There is no clear consensus on the administration of anti-neoplastic agents to patients on peritoneal dialysis. Dose adjustments to prevent serious adverse events are still not established. Thus, the aim of this study was to systematically review current evidence on the use of systemic oncology therapies in peritoneal dialysis.

METHODS

A systematic review was conducted using PubMed, Scopus, and Cochrane. All relevant data was collected, including clinical and pharmacokinetic parameters, with comparison to subjects with normal renal function.

RESULTS

Sixteen studies were included. All were case reports. Eighteen types of anti-cancer drugs were reviewed. Multiple adverse events and altered pharmacokinetics were reported.

CONCLUSION

Data concerning the use of anti-neoplastic drugs in patients on peritoneal dialysis are still sparse. The elimination of anti-cancer agents seems often altered in such patients, resulting in serious adverse events. Based on the available evidence, we suggest the need for dose adjustment of each drug.

Exploring the new horizons of drug repurposing: A vital tool for turning hard work into smart work

Drug discovery and development are long and financially taxing processes. On an average it takes 12-15 years and costs 1.2 billion USD for successful drug discovery and approval for clinical use. Many lead molecules are not developed further and their potential is not tapped to the fullest due to lack of resources or time constraints. In order for a drug to be approved by FDA for clinical use, it must have excellent therapeutic potential in the desired area of target with minimal toxicities as supported by both pre-clinical and clinical studies. The targeted clinical evaluations fail to explore other potential therapeutic applications of the candidate drug. Drug repurposing or repositioning is a fast and relatively cheap alternative to the lengthy and expensive de novo drug discovery and development. Drug repositioning utilizes the already available clinical trials data for toxicity and adverse effects, at the same time explores the drug's therapeutic potential for a different disease. This review addresses recent developments and future scope of drug repositioning strategy.

Retinoic Acid Induces Apoptosis of Prostate Cancer DU145 Cells through Cdk5 Overactivation

Retinoic acid (RA) has been believed to be an anticancer drug for a long history. However, the molecular mechanisms of RA actions on cancer cells remain diverse. In this study, the dose-dependent inhibition of RA on DU145 cell proliferation was identified. Interestingly, RA treatment triggered p35 cleavage (p25 formation) and Cdk5 overactivation, and all could be blocked by Calpain inhibitor, Calpeptin (CP). Subsequently, RA-triggered DU145 apoptosis detected by sub-G1 phase accumulation and Annexin V staining could also be blocked by CP treatment. Furthermore, RA-triggered caspase 3 activation and following Cdk5 over-activation were destroyed by treatments of both CP and Cdk5 knockdown. In conclusion, we report a new mechanism in which RA could cause apoptosis of androgen-independent prostate cancer cells through p35 cleavage and Cdk5 over-activation. This finding may contribute to constructing a clearer image of RA function and bring RA as a valuable chemoprevention agent for prostate cancer patients.

Multiple cranial neuropathy and intracranial hypertension associated with all-trans retinoic acid treatment in a young adult patient with acute promyelocytic leukemia

All-trans retinoic acid (ATRA) induces complete remission in 64-100 % of patients with acute promyelocytic leukemia (APL), and is considered to be a safe agent. Pseudotumor cerebri is a neurological side effect of ATRA reported in pediatric patients, and which is characterized by raised cerebrospinal fluid pressure in the absence of any intracranial pathology or secondary causes of intracranial hypertension. Involvement of cranial nerves other than II and VI is very uncommon in idiopathic intracranial hypertension (IIH); peripheral facial nerve palsy is exceptional and has rarely been described in the context of treatment with ATRA. We describe the case of a 15-year-old female patient with APL who developed an IIH and involvement of cranial nerves (bilateral papilledema, left facial and right sixth nerves) after receiving induction therapy including ATRA. Viral infections and other causes of secondary cranial nerve lesions were excluded. Symptoms completely subsided with the temporary withdrawal of ATRA and did not recur after reintroducing the drug. To date, the patient has managed to receive the treatment as per protocol. In conclusion, we report an atypical presentation of IIH that merits consideration, especially with respect to young patients with APL receiving ATRA; our most important observation is that the drug could be safely reintroduced once the symptoms had resolved.

Enhanced anticancer activity of glutamate prodrugs of all-trans retinoic acid

Objectives

All-trans retinoic acid (ATRA), an active metabolite of vitamin A, is widely used in the treatment of acute promyelocytic leukaemia and myelodysplastic syndrome. However, its high lipophilicity is thought to be responsible for the slow dissolution and low bioavailability following oral administration. In order to obtain compounds with better solubility characteristics to improve the transportation and bioavailability of ATRA, derivatives of ATRA containing glutamic acid or its sodium salt were synthesised.

Methods

The ATRA derivatives synthesised – all-trans retinoyl glutamate (RAE) and all-trans retinoyl sodium glutamate (RAENa2) – were characterised in terms of melting point, optical rotation, mass spectrometry, NMR and partition coefficient. A liposomal preparation formed from RAE was characterised by particle size and zeta potential. The anti-tumour activity of RAE and RAENa2 was compared with that of ATRA in mice bearing S180 tumours and their effects on the cell cycle were determined in human pro-myelocytic leukaemia HL-60 cells.

Key findings

RAE and RAENa2 were more active than ATRA against tumour growth. Flow cytometry indicated that RAE and RAENa2 induced HL-60 cell cycle arrest, similar to ATRA. DNA fragmentation studies suggested that apoptosis may be one of the mechanisms responsible for the anti-tumour activities.

Conclusions

The two derivatives of ATRA, RAE and RAENa2, exhibited improved aqueous solubility and were more effective in mice bearing S180 tumours.

Overexpression of BP1, a homeobox gene, is associated with resistance to all-trans retinoic acid in acute promyelocytic leukemia cells

BP1, a homeobox gene, is overexpressed in the bone marrow of 63% of acute myeloid leukemia patients. In this study, we compared the growth-inhibitory and cyto-differentiating activities of all-trans retinoic acid (ATRA) in NB4 (ATRA-responsive) and R4 (ATRA-resistant) acute promyelocytic leukemia (APL) cells relative to BP1 levels. Expression of two oncogenes, bcl-2 and c-myc, was also assessed. NB4 and R4 cells express BP1, bcl-2, and c-myc; the expression of all three genes was repressed after ATRA treatment of NB4 cells but not R4 cells. To determine whether BP1 overexpression affects sensitivity to ATRA, NB4 cells were transfected with a BP1-expressing plasmid and treated with ATRA. In cells overexpressing BP1: (1) proliferation was no longer inhibited; (2) differentiation was reduced two- to threefold; (3) c-myc was no longer repressed. These and other data suggest that BP1 may regulate bcl-2 and c-myc expression. Clinically, BP1 levels were elevated in all pretreatment APL patients tested, while BP1 expression was decreased in 91% of patients after combined ATRA and chemotherapy treatment. Two patients underwent disease relapse during follow-up; one patient exhibited a 42-fold increase in BP1 expression, while the other showed no change. This suggests that BP1 may be part of a pathway involved in resistance to therapy. Taken together, our data suggest that BP1 is a potential therapeutic target in APL.

Chemoprevention of 4-NQO-induced oral carcinogenesis by co-administration of all-trans retinoic acid loaded microspheres and celecoxib

All-trans retinoic acid (atRA) is one of the most potential chemopreventive agents for head and neck squamous cell carcinoma (HNSCC). However, the induced metabolism of atRA by cytochrome P450s in the liver limits its clinical applications. To overcome such limitation, we had developed atRA-loaded microspheres designed to release atRA for a long period. Unfortunately, the atRA-loaded microspheres severely induced inflammatory responses: that is, atRA released from the microspheres significantly induced the proliferation of fibroblasts and collagen deposition, thereby causing a permeability barrier for drugs from entering the blood stream. In the present study, the effects of celecoxib as an anti-inflammatory drug are investigated when it is concurrently used with atRA-loaded microspheres to treat 4-NQO-induced oral carcinogenesis. We investigated if it might influence the plasma concentration of atRA and its metabolism by preventing the fibroblast proliferation and collagen deposition, reduce the toxicity level of atRA, and improve the chemopreventive efficacy of atRA-loaded microspheres. The concurrently administered celecoxib prevented inflammatory responses and suppressed the number of fibroblasts and collagen deposition in the fibrous capsules for 14 days. The atRA concentration in plasma was also increased and the metabolism of atRA was significantly decreased within 2 weeks. In the 4-NQO-induced oral carcinogenesis study, the incidence of invasive SCC was above 44% when F344 rats were treated with atRA-loaded microspheres. However, the treatment using atRA-loaded microspheres and celecoxib concurrently could reduce the incidence of invasive SCC up to 28%, and three of 25 rats were found to have no tongue lesions. In conclusion, the concurrent use of celecoxib could maintain the atRA concentration in plasma at a higher level while reducing its metabolism by preventing inflammatory responses, thereby improving their chemopreventive effects against 4-NQO-induced oral carcinogenesis.

Inhibition of tumor growth by biodegradable microspheres containing all-trans-retinoic acid in a human head-and-neck cancer xenograft

Retinoids play essential roles in the regulation of cell differentiation and in the proliferation of various epithelial tissues, and atRA is one such active metabolite of retinoids. However, despite the known functions of atRA, its clinical applications are limited due to the induced metabolism by the specific cytochrome P-450s in the liver. To overcome the limitation, parenteral administration of atRA-loaded biodegradable microspheres, the PDLLA/PLE microspheres containing atRA, was suggested previously. We evaluated chemotherapeutic efficacy of atRA-loaded microspheres in a human head-and-neck xenograft/nude mouse model. When atRA-loaded microspheres were administered s.c. at 200 mg/kg body weight to athymic nude mice, plasma concentration of atRA could be maintained in a range of 1.2 to 3.7 x 10(-8) M for 4 weeks. As a result, the tumor volume of human head-and-neck cancer was reduced compared to the control group by 51.3% (p < 0.01) at 14 days and by 49.2% (p < 0.05) at 28 days.

In vivo biocompatibility studies of poly(D,L-lactide)/poly(ethylene glycol)-poly(L-lactide) microspheres containing all-trans-retinoic acid

Biocompatibility studies of all-trans-retinoic acid (RA)-loaded microspheres were carried out after they were subcutaneously injected into rats. To characterize the inflammatory response to these microspheres, tissue reactions at the implantation site and cell types in the interstices of the microspheres were evaluated for 180 days. On the 15th day, the cross-sectional area of the fibrous capsules surrounding the implantation site of the RA-loaded microspheres was four times larger than that of the control microspheres. The size of the fibrous capsules surrounding the implantation site of the RA-loaded microspheres decreased significantly over a period of 75 days, while the size of the fibrous capsules surrounding the implantation site of the control microspheres remained almost constant throughout the entire course of 180 days. The tissue response to the RA-loaded microspheres was more intensified by the increased extensive cellular infiltration of macrophages, granulation tissue, and fibrosis than that to the control microspheres. The difference in the inflammatory response between the RA-loaded microspheres and the control microspheres was significant for 75 days after implantation. It was suggested that the released RA from the microspheres stimulated inflammatory responses. However, no further enhanced inflammation reactions were detected after RA had been completely released from the microspheres.

Transcript profiling of cytochrome P450 genes in HL-60 human leukemic cells: upregulation of CYP1B1 by all-trans-retinoic acid

All-trans-retinoic acid (ATRA) is used in the treatment of promyelocytic acute leukemia. The biotransformation of this drug is catalyzed by various cytochrome P450 (CYP) enzymes, but relatively little is known about the effect of ATRA on CYP enzyme expression in leukemic cells. In the present study, we conducted transcript profiling of CYP and related genes in cultured HL-60 human promyelocytic leukemic cells and determined the effect of ATRA on the expression of these genes. Reverse transcription-polymerase chain reaction (RT-PCR) analysis with a block-cycler indicated the presence of CYP1B1 but not CYP1A1, CYP2B6, CYP2C8, CYP2C9, CYP3A4, CYP3A5, or CYP26A1 transcript in cultured HL-60 cells. ATRA treatment (0.1-40 microM for 3 days) increased CYP1B1 mRNA levels by up to 3 fold, as determined by a quantitative real-time PCR method. The same ATRA treatment also resulted in the detection of CYP26A1 but not CYP1A1, CYP2B6, CYP2C8, CY2C9, CYP3A4, or CYP3A5 mRNA. Additional experiments showed that phenobarbital increased CYP2B6 mRNA expression and that pregnane X receptor (PXR) but not constitutive androstane receptor (CAR) was detected in HL-60 cells. Overall, our novel findings indicate the upregulation of CYP1B1 by ATRA in HL-60 human promyelocytic leukemic cells shown for the first time to express PXR but not CAR mRNA.

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.

Long-term delivery of all-trans-retinoic acid using biodegradable PLLA/PEG-PLLA blended microspheres

All-trans-retinoic acid (atRA) has been proved to be effective against several malignancies in human clinical trials. However, in many patients who were treated with atRA, the cancer relapsed after a brief remission. One reason for such relapse is that atRA is metabolized by specific P450s that are induced in the liver during prolonged atRA treatments. In order to overcome such a drawback of atRA, we prepared biodegradable microspheres to provide continuous release of atRA for a long period of time. These biodegradable microspheres were prepared by poly(L-lactide) (PLLA) and polyethylene glycol (PEG)-PLLA diblock copolymers (PLE) in various blending ratios to control the release rate of atRA. As the PLE content in microsphere was increased, the density of the hydrophilic PEG block of PLE on microsphere surfaces increased and the microspheres were dispersed well in PBS without any surfactants. Various release patterns of atRA were obtained according to PLE and atRA contents in the microspheres. Especially, the pseudo-zero-order release profiles were observed for 5 weeks when the contents of PLE and atRA in the microspheres were above 4 wt.%.

Induction of differentiation and apoptosis- a possible strategy in the treatment of adult acute myelogenous leukemia

A differentiation block with accumulation of immature myeloid cells characterizes acute myelogenous leukemia (AML). However, native AML cells often show some morphological signs of differentiation that allow a classification into different subsets, and further differentiation may be induced by exposure to various soluble mediators, e.g., all trans-retinoic acid (ATRA) and several cytokines. Combination therapy with ATRA and chemotherapy should now be regarded as the standard treatment for the acute promyelocytic leukemia variant of AML. Several agents can induce leukemic cell differentiation for other AML subtypes, although these effects differ between patients. Differentiation may then be associated with induction of apoptosis, and differentiation-inducing therapy may therefore become useful in combination with intensive chemotherapy to increase the susceptibility of AML blasts to drug-induced apoptosis. However, it should be emphasized that differentiation and apoptosis can occur as separate events with different regulation in AML cells, and future studies in AML should therefore focus on: A) the identification of new agents with more predictable effects on differentiation and apoptosis; B) the use of clinical and laboratory parameters to define new subsets of AML patients in which differentiation/apoptosis induction has a predictable and beneficial effect, and C) further characterization of how AML blast sensitivity to drug-induced apoptosis is modulated by differentiation induction.

New strategies for the treatment of acute myelogenous leukemia: differentiation induction--present use and future possibilities

A differentiation block and an accumulation of immature myeloid cells characterize acute myelogenous leukemia (AML). However, native AML cells usually show some morphological signs of differentiation that allow a classification into different subsets, and further differentiation may be induced by exposure to various soluble mediators, for example, all-trans retinoic acid (ATRA) and several cytokines. Combination therapy with ATRA and chemotherapy should now be regarded as the standard treatment of the acute promyelocytic leukemia (APL) variant of AML. Although several agents can also induce leukemic cell differentiation for other AML subgroups, in vitro studies as well as clinical data have demonstrated that these agents often have heterogeneous effects on the leukemic progenitors. This makes the clinical impact of differentiation induction therapy for individual patients difficult to predict. However, differentiation induction should be regarded as a promising therapeutic approach, especially as a part of immunotherapy or in combination with intensive chemotherapy to increase the susceptibility of AML blasts to drug-induced apoptosis. Although the morphology-based French-American-British classification was used to identify APL as an AML subset that required a special treatment, it seems unlikely that this classification alone can be used to identify new subsets of AML patients with special therapeutic requirements. Future studies on differentiation induction in AML should therefore focus on A) the identification of therapeutic agents with more predictable effects; B) the use of clinical and laboratory parameters to define new subsets of AML patients in which differentiation induction has a predictable and beneficial effect, and C) the characterization of how AML blast sensitivity to drug-induced apoptosis is altered by differentiation induction.

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.

all-Trans-retinoic acid reduces neointimal formation and promotes favorable geometric remodeling of the rat carotid artery after balloon withdrawal injury

BACKGROUND

The multifactorial and unpredictable nature of human restenosis will probably necessitate interventional strategies that target multiple processes involved in acute vascular narrowing. Retinoids (eg, all-trans-retinoic acid, atRA) represent a growing class of pleiotropic biological response modifiers with demonstrable efficacy in managing several pathological conditions. In this report, we have initiated studies to examine the hypothesis that atRA limits neointimal formation after experimental vascular injury.

METHODS AND RESULTS

Rats were predosed with atRA (30 mg . kg-1 . d-1 PO) or corn oil 4 days before balloon withdrawal injury (BWI) of the left common carotid artery and continued on this drug regimen for an additional 14 days. High-performance liquid chromatographic analysis documented therapeutic levels of atRA in serum and vascular tissue. atRA depressed peak DNA synthesis in the tunica media of BWI vessels (P<0.05). Histomorphometry revealed atRA-mediated reductions in neointimal area, neointimal cell number, and intimal/medial area ratio as well as significant increases in vessel wall perimeter (P<0. 05). Such changes in vascular architecture contributed to a 35% to 37% increase in the luminal area of BWI vessels exposed to atRA (P<0. 005 compared with controls).

CONCLUSIONS

atRA reduces neointimal mass and elicits favorable geometric remodeling of the injured rat carotid artery.

Clinical pharmacokinetics of tretinoin

Recent reports of the dramatic antitumour effect of tretinoin (all-trans retinoic acid) in patients with acute promyelocytic leukaemia (APL) have generated a great deal of interest in the use of this drug as a chemopreventive and therapeutic agent. However, the biological efficacy of tretinoin is greatly impaired by (presumably) an induced hypercatabolism of the drug leading to reduced tretinoin sensitivity and resistance. Several pharmacokinetic studies have shown that plasma drug exposure [as measured by the plasma area under the concentration-time curve (AUC infinity)] declines substantially and rapidly when the drug is administered in a long term daily tretinoin regimen. These observations led to the hypothesis that the rapid development of acquired clinical resistance to tretinoin may have a pharmacological basis and result from an inability to present an effective drug concentration to the leukaemic cells during continuous treatment. The principal mechanisms proposed to explain the increased disappearance of tretinoin from plasma include: (i) decreased intestinal absorption; (ii) enhanced enzymatic catabolism; and (iii) the induction of cytoplasmic retinoic acid binding proteins (CRABP), which leads to increased drug sequestration. The most favoured explanation is that continuous tretinoin treatment acts to induce drug catabolism by cytochrome P450 (CYP) enzymes. Several strategies aimed at preventing or overcoming induced tretinoin resistance have been, and are being, planned. These strategies include intermittent dose administration, administration of pharmacological inhibitors of CYP oxidative enzymes, combination with interferon-alpha and intravenous administration of liposome-encapsulated tretinoin. As these strategies are now under investigation and the number of patients enrolled is small, further studies are needed to determine the efficacy and toxicity of these new schedules of drug administration. In this article we provide an overview of the relevant aspects of tretinoin physiology and pharmacokinetics, and summarise the current status of knowledge to help in the better optimisation of tretinoin administration.

Acute promyelocytic leukaemia in the all trans retinoic acid era

Bleeding diathesis is a common complication of acute promyelocyctic leukaemia (APL). Multiple haemostatic defects are found in most patients with APL, which often worsen following cytoreductive chemotherapy. Besides thrombocytopenia, most patients develop disseminated intravascular coagulation, systemic fibrinolysis or both. A major aim in treating haemostatic defects of APL is to prevent death or disability from bleeding until chemotherapy clears the malignant promyelocytes from the blood and bone marrow. The therapeutic options are discussed in this review and practical guidelines for treatment are outlined.