1. Introduction
Extra-virgin olive oil (EVOO) is one of the key components in the Mediterranean diet. It has been pointed as a contributing factor towards the epidemiologically documented favorable health benefits in Mediterranean populations due to the presence of minor phenolic ingredients of EVOO [
1,
2]. EVOO has gained notable scientific attention focused on the beneficial effects of its phenolic components, including anti-inflammatory, antioxidant, and antimicrobial activities. Several studies have already shown the correlation of a high intake of EVOO with a lower incidence of colon and breast cancers, osteoporosis, cardiovascular, metabolic, and Alzheimer’s disease [
3,
4,
5].
(–)-Oleocanthal (OC, decarboxymethyl ligstroside aglycone) is a unique phenolic secoiridoid exclusively occurring in EVOO. OC provides the irritative, bitter, pharyngeal pungent, and astringent taste of EVOO [
6,
7]. OC was first discovered in 1992, and its chemical structure was reported later [
8,
9]. In addition, in 2005, the Beauchamp group suggested the name oleocanthal with the suffix “
oleo” for olive, “
canth” for stinging, and “
al” for its aldehydes, and reported its potent anti-inflammatory activity [
10]. Several literatures later documented the anti-inflammatory, antioxidant, antimicrobial, anticancer, and neuroprotective activities of OC [
1].
The anti-inflammatory potency of OC was comparable with the non-steroidal anti-inflammatory drugs (NSAID) like ibuprofen by inhibiting COX-1 and COX-2. OC also exhibited an irritant taste similar to ibuprofen [
10,
11]. The OC dialdehydic functionality was found to be the main pharmacophore activating the transient receptor potential cation channel subtype A1 (TRPA1) receptor, which translates the irritative and pungent taste sensation [
12,
13]. Distinct OC nociceptors were suggested in the oral cavity-oropharyngeal region.
OC inhibited the LPS-mediated upregulation of proinflammatory signaling molecules, including interleukin-1β (IL-1β), IL-6, macrophage inflammatory protein-1α (MIP-1α), tumor necrosis factor-α (TNF-α), and granulocyte-macrophage-colony-stimulating factor (GM-CSF). OC showed neuroprotective effects against Alzheimer’s disease by altering the structure and function of β-amyloids, tau phosphorylation, and reducing the inflammation of astrocytes [
14,
15,
16,
17]. On the other hand, numerous studies showed that OC played a role in inducing apoptosis and inhibiting the migration, angiogenesis, and metastasis of cancerous cell lines originating from hepatocellular carcinoma [
18], prostate cancer [
19], human melanoma [
20], and non-melanoma skin cancers [
21], colorectal carcinoma [
22], and breast cancer (BC) [
7,
23]. OC exhibited its anti-BC and anti-prostate cancer effects through competitive inhibition of c-MET kinase ATP activation [
7,
19]. OC acted via AMPK inhibition by the suppression of MIP-1α in multiple myeloma [
22].
Despite the documented in vitro and in vivo bioactivities, there is no reported pharmaceutically acceptable oral dosage form of OC to date. Thus, an acceptable OC pharmaceutical oral dosage form is needed to facilitate its application as a potential nutraceutical for diverse therapeutic applications. On the other hand, appropriate OC oral formulation must carefully consider its chemical instability, due to the reactive aldehyde and ester groups, irritative and pungent taste, and poor water solubility, and maintain its in vivo bioactive potency. The effervescent composition is a convenient dosage form producing CO
2 effervescence as an underused formulation approach in which the Active Pharmaceutical Ingredient (API) is administered in aqueous solution form. CO
2 effervescence imparts taste masking of the pungent and bitter taste of APIs [
24]. Interestingly, the OC irritative taste was not correlated with the acid-induced irritation, and hence, OC was proven not to exert generalized acid-sensing irritation [
13]. The taste intensity ratings of OC and CO
2 were not correlated, and therefore, they were hypothesized to have independent irritative taste mechanisms [
13]. This may facilitate the use of CO
2 for effective OC taste masking. Furthermore, the effervescence composition is obtained by reacting acids like citric and/or tartaric acids with bases like carbonates or bicarbonates in the presence of water, which releases CO
2 to provide acceptable carbonated or sparkling drinkable liquid. Due to the liberation of CO
2, the dissolution is expected to improve by the imparted acidity [
25]. In addition, the effervescent formulation provides functional and consumer advantage over conventional pharmaceutical dosage forms. Therefore, effervescent formulations are favored by geriatric and pediatric patient populations, in addition to adult populations with gag reflexes and/or swallowing difficulty [
26].
Thus, the main objectives of this study are: (i) to develop an effervescent OC dosage form with effective taste masking and enhanced dissolution; (ii) to assess the efficacy of the formulated effervescent powder against the growth of the estrogen receptor positive/human epidermal receptor-2 positive (ER+/HER2+) BC in vitro and in vivo in nude mouse xenograft models; (iii) to determine the ER+/HER2+ BC locoregional recurrence inhibitory efficacy of the formulated effervescent powder in an orthotopic xenograft mouse model after primary tumor surgical excision.
2. Materials and Methods
2.1. Chemicals, Reagents, and Antibodies
Citric acid, tartaric acid, and sodium bicarbonate were purchased from Fisher Scientific (New Brunswick, NJ, USA). In addition, mannitol and aerosil-200 were acquired from Sigma Aldrich (St. Louis, MO, USA). All primary and secondary antibodies were purchased from Cell Signaling Technology (Beverly, MA, USA), unless otherwise stated. The CA 15-3 (Human) ELISA Kit (Catalog number KA0206) was purchased from Abnova (Walnut, CA, USA).
2.2. (–)-Oleocanthal Isolation from Extra-Virgin Olive Oil Samples
Oleocanthal was extracted from EVOO (The Governor, batch #: 5-214000-242017) using a liquid–liquid extraction method where the successful capacity of water was used to efficiently extract OC as its monohydrate and utilize its self-emulsifying tendency [
26]. Selective resin entrapment enabled the water and residual fatty acids elimination and recovered OC in high-yield and good purity [
26]. Final purification was conducted on Sephadex LH20, isocratic elution with CH
2Cl
2. The purity of OC (>99%) was assessed on a Phenomenex Cosmosil 5C18-AR-II column (250 mm × 4.6 mm, 5 µm; Phenomenex Inc., Torrance, CA, USA) using 1:1 H
2O–CH
3CN isocratic elution as a mobile phase. The purity and chemical identity of OC was further confirmed by NMR analysis on a JEOL Eclipse ECS-400 NMR spectrometer. The q
1H NMR confirmed >99% purity of OC, which was stored at −20 °C in a freezer in an amber glass vial after purging out air with N
2 gas [
26].
2.3. Determination of Effervescent Components
The effervescent ingredients and their ratios were optimized depending on acid–base interaction. Different amounts of citric acid, tartaric acid, and sodium bicarbonate were mixed and poured into 100 mL of water, followed by the immediate measurement of effervescence time and pH. The effervescence time and pH were evaluated by using a standard stopwatch and calibrated pH meter (pH meter, OAKTON Instruments, Vernon Hills, IL, USA), respectively. Each experiment was repeated three times.
2.4. Preparation of Effervescent Powder
OC was mixed with flavor and then adsorbed by the adsorbing agent Aerosil 200 (Mixture-1). On the other hand, the required quantities of citric acid, sodium bicarbonate, and tartaric acid were uniformly blended together. Mannitol was then added, mixed with the blended powder of acids and sodium bicarbonate, and passed through a 40-mesh screen (mixture-2). Mixture-2 was added to mixture-1 in different ratios, thoroughly blended, and passed through a 40-mesh screen. Formulation randomization was based on various ratios of citric, tartaric, and NaHCO3 contents. One ingredient was fixed, and others optimized in various ratios to create various formulations. Different formulations were prioritized based on the optimal effervescent time and pH results.
2.5. Determination of Solution pH
Each effervescent formulation powder was dissolved into 300 mL of deionized distilled water, and the pH of the solution was measured by using a pH meter. Each experiment was repeated three times.
2.6. Determination of Effervescence Time
Each effervescent formulation powder was poured into 300 mL of deionized distilled water, and in vitro effervescence time was determined by a stopwatch. Each experiment was repeated three times.
2.7. Determination of CO2 Content
A glass beaker containing 80 mL of deionized distilled water was covered with a polyethylene cover, which was cut in a rectangular shape (2.9 × 0.6 cm) so that the effervescent powder easily passed through the beaker [
27]. This covered glass beaker was kept on an analytical balance at room temperature and the weight was recorded. A total of 1 g of each effervescent formulation powder was dropped into the beaker and left for 2 min. The final weight loss was recorded, representing the CO
2 amount in mg lost during the effervescent reaction.
2.8. Flow Properties of Powder
2.8.1. Angle of Repose (θ)
The static angle of repose was determined by the fixed funnel method, which is exhibited as the maximum possible angle between the surface of a powder pile or powder and the horizontal plane. The powder was allowed to pass through a funnel fixed to a stand at a definite height. Each experiment was repeated three times. The angle of repose (
θ) was calculated by measuring the height (
h) and radius (
r) of the formed powder heap using the formula [
28]:
2.8.2. Compressibility Index
The flowability of powder was measured by comparing the powder bulk density (
Dp) and tapped density (
Dt). Each experiment was repeated three times. The compressibility index percentage was calculated using the formula:
2.8.3. Hausner’s Ratio
Hausner’s ratio is an important property to determine the flow property of powder. Each experiment was repeated three times. This can be calculated using the following formula:
with bulk density (
Dp) and tapped density (
Dt) of powder.
2.9. Fourier Transform Infrared Spectroscopy
The Fourier transform infrared (FT-IR) spectra of all formulations, plain non-formulated OC, and placebo carrier were recorded using a PerkinElmer Spectrum-Two™ FT-IR spectrometer (Waltham, MA, USA). Samples were analyzed using a diffuse reflectance cell, without prior sample preparation, by directly compressing on the ATR crystal under appropriate compression conditions and scanned at a resolution of 4 cm−1 using the absorbance over the wavenumber range from 400–4000 cm−1. Each sample IR spectrum was acquired three times.
2.10. Thermal Analysis of the Effervescent Powder by Differential Scanning Calorimetry (DSC)
The thermal analysis of each of the five effervescent powders was performed using a TA 2920 modulated differential scanning calorimeter (DSC, TA Instruments-Waters LLC, New Castle, DE, USA) under nitrogen atmosphere at a flow rate of 25 mL/min. An accurately weighed 5 mg of each sample was hermetically sealed in an aluminum crimp pan and heated from 0–350 °C at a temperature acceleration rate 10 °C/min. The sample was cooled using a DSC refrigerated cooling system (TA Instruments). Melting endotherms were analyzed by the universal analysis 2000 software, V 4.2 (TA Instruments-Waters LLC, New Castle, DE, USA) and compared with the placebo.
2.11. Scanning Electron Microscopy (SEM) and Confocal Laser Scanning Microscopy (CLSM)
EF-2 was placed on a brass stub with the application of double-sided adhesive tape, and then coated in a vacuum chamber with a thin layer of gold for 30 seconds (Sputter coater, Edwards, S150A, England, UK) to prepare it as electrically conductive. The pictures were taken at an excitation voltage of 20 kV (Electron probe microanalyzer, JEOL, JXA-840A, Tokyo, Japan) to check the surface morphology of the selected formula. A confocal laser scanning microscope was used to examine the surface morphology of EF-2. A Biorad MRC 1024 Laser Scanning Confocal Imaging System (Hemel Hempstead, UK), equipped with an argon ion laser (American Laser Corp, Salt Lake City, UT, USA) and a Zeiss Axiovert 100 microscope (Carl Zeiss, Oberkochen, Germany) was used to examine the effervescent formulation.
2.12. In-Vitro Dissolution Study
The dissolution profile of each of plain non-formulated OC and EF-2 powder formulation was carried out in a 100 mL simulated gastric fluid (SGF, pH 1.2), without enzymes, and simulated intestinal fluid (SIF, pH 6.8), without enzymes. This test was conducted by utilizing a USP type II dissolution apparatus (VK 7000, Varian Inc., Cary, NC, USA) at a paddle speed of 100 rpm. The temperature of the dissolution medium was maintained at 37 ± 5 °C using a Varian VK750 heater (Varian Inc., Cary, NC, USA). The effervescent powder of 10 mg of OC was packed into a size 00 transparent hydroxypropyl methylcellulose (HPMC) capsule and inserted into the medium by using an individual sinker. About 1 mL aliquot sample was withdrawn at time intervals of 2, 5, 10, 20, 30, 40, and 60 min, which was replaced with an equivalent amount of fresh dissolution medium. The collected samples were filtered, and OC content was analyzed by an HPLC system equipped with a UV/Visible variable wavelength detector at λmax 230 nm (Shimadzu Scientific Instrument, Japan). Each experiment was repeated three times. A 20 μL sample was then injected into the Eclipse YD5 C18-RP analytical column (4.6 mm × 15 cm) at a flow rate of 1.0 mL/min. Acetonitrile and water (50:50) mixture was used isocratically as a mobile phase. Data acquisition and analysis were performed using Lab Solution™ chromatography software.
2.13. Taste Assessment Using an Electronic Tongue
The ASREE electronic tongue, generated by Alpha MOS, is an adequate tool for assessing taste sensing and therefore used to compare the effectiveness of effervescent formulation to mask the irritant OC taste with OC and a placebo. This test measures the comparative percentage between two principal component analyses (PCA1-PCA2) to describe total data variation by processing data acquired by the e-tongue from seven different taste sensors to two-dimensional data. The assays were analyzed on the ASTREE e-tongue system equipped with the array consisting of an Alpha MOS sensor set #2 for pharmaceutical analysis, which composed of 7 specific sensors (ZZ, AB, GA, BB, CA, DA, JE) [
29,
30]. The system composed of a 48-position autosampler and 25 mL capacity beakers for sampling, an array of liquid sensors, an electronic unit for sensor data acquisition using multidimensional chemometric statistics-Alpha Soft v15.0 software, and the acquisition times were fixed at 120 seconds. The euclidian distances and percentage discrimination index (DI) between samples were calculated to assess taste proximity; the lower the distance, the closer the taste. Data generated by the ASTREE system processed taste analysis was conducted in artificial saliva without enzymes, pH 6.8; X1, comparing a vehicle control, non-formulated OC; X2, effervescent formulation; X3, placebo formulation; X4. Discrimination index (DI in %) was determined for each formulation and placebo pair. The closer the DI to 100%, the greater the distance between the centers of gravity and the smaller the dispersion within groups.
2.14. Animal Preference Experiment
Male and female 6-week old Swiss Albino mice weighing 20–25 g were obtained from Envigo (Indianapolis, IN, USA). All animal experiments were approved by the Institutional Animal Care and Use Committee (IACUC), University of Louisiana at Monroe, protocol number 17OCT-KES-01, approved on 17 October 2017, with good animal practice defined by the NIH guidelines. Mice were acclimated to the University of Louisiana-Monroe, College of Pharmacy animal housing facility and maintained under clean room conditions in sterile filter top cages using Alpha-Dri bedding and high efficiency particulate air-filtered ventilated plastic racks at 25 °C, 55–65% relative humidity, and a 12 h light/dark cycle for a week before experiments. Mice had free access to purified drinking water and pelleted rodent chow (no. 7012, Envigo/Teklad, Madison, WI, USA). Taste preferences were assessed for 48 h using a two-bottle choice test [
31] for the following samples:
(i) Plain, non-formulated OC, (ii) effervescent formulation EF-2 at a daily oral dose of 10 mg OC/kg of body weight, and (iii) placebo formulation. A total of 30 mice (15 males and 15 females) were randomized into 3 groups for each gender, n = 5 mice. The mice had access to two drinking bottles; one contained distilled water, and the other contained either a plain non-formulated OC sample, EF-2, or placebo formulation for 48 h. The positions of both drinking bottles were switched every 24 h. The volume consumed of each bottle was recorded (using a volumetric level scale to the nearest 0.1 mL) at the beginning and end of the experiment after 48 h. The total fluid intake of each sample was obtained by adding the volume intakes for each sample. Percentage preference was calculated as intake of the solution of sample bottle divided by the total fluid intake.
2.15. Cell Lines and Culture Conditions
The human BC cell lines MDA-MB-231 and BT-474 were purchased from ATCC and maintained in RPMI-1640 supplemented with 10% FBS, 100 U/mL penicillin, 0.1 mg/mL streptomycin in a humidified atmosphere of 5% CO2 at 37 °C. All cells were maintained at 37 °C in an environment of 95% air and 5% CO2 in a humidified incubator.
MDA-MB-231 or BT-474 cells were plated at a density of 1 × 10
4 cells per well (6 wells/group) in 96-well culture plates and maintained in RPMI-1640 media supplemented with 10% FBS, which allowed to adhere overnight. The next day, cells were washed with phosphate buffer saline (PBS), divided into different treatment groups and then given various concentrations of EF-2 formulation or placebo treatment media for 48 h. Viable cells count was determined using the 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl tetrazolium bromide (MTT) colorimetric assay, whereas the optical density of each sample was measured at 570 nm on a microplate reader (BioTek, VT, USA). Furthermore, the number of cells/well was calculated against a standard curve prepared by plating various concentrations of cells, which were measured by using a hemocytometer at the beginning of each experiment [
32].
2.16. Effects of EF-2 on BT-474 Nude Mice Tumor Xenograft Progression and Recurrence Models
The inhibitory effects of EF-2 formulation administration against the growth and recurrence of the human HER2+-ER+ BC cells - BT-474 cells orthotopically xenografted in nude mice were assessed. Foxn1nu/Foxn1+, 4–5 week old, female athymic nude mice were purchased from Envigo (Indianapolis, IN). All animal experiments were approved by the Institutional Animal Care and Use Committee (IACUC), University of Louisiana at Monroe, protocol number 18 MAY-KES-02, approved on 18 May 2018, with good animal practice defined by the NIH guidelines. Mice were acclimated to the University of Louisiana-Monroe, College of Pharmacy animal housing facility and maintained under clean room conditions in sterile filter top cages using Alpha-Dri bedding and high efficiency particulate air-filtered ventilated racks at 25 °C, 55–65% relative humidity, and a 12 h light/dark cycle for a week before experiments. Husk and excreta were taken away from the cages daily. Mice had free access to purified drinking water and pelleted rodent chow (no. 7012, Envigo/Teklad, Madison, WI). Animals were orally dosed daily at 10 mg/kg OC in EF-2 dissolved in sterile normal saline using 18G plastic (PTFE) with stainless steel bite protector oral feeding needles (VWR, Suwanee, GA, USA).
2.16.1. Tumor Growth Inhibition
BT-474 human BC cells were cultured and resuspended in serum-free RPMI-1640 medium and Matrigel with a 50:50 ratio. In addition, after anesthesia, cell suspensions (5 × 106 cells/60 µL) were subcutaneously inoculated into the second mammary gland fat pad just beneath the nipple of each animal to generate orthotopic breast tumors. Mice were then randomly divided into two groups: i) the placebo control group (n = 5), and ii) the EF-2-treated group (n = 5), at a dose of 10 mg OC/kg. Oral treatments, placebo control, or EF-2 started on the tumor cells inoculation day and continued daily thereafter. EF-2 was dissolved in 1 mL of water at a concentration of 1 mg/mL and immediately given fresh to the mice every day. The mice were monitored daily by measuring tumor volume, body weight, and clinical observation. Tumor volume (V) was calculated by V = L/2 × W2, where L was the length and W was the width of tumors. At the end of the experiment, the primary tumors were surgically excised and weighed.
2.16.2. Tumor Recurrence Inhibition
To investigate the efficacy of EF-2 against recurrence tumor, animals used in the previous growth models were used. Once the average tumor volume in the control mice group reached ~1000 mm3, mostly on day 27, animals were anesthetized with i.p. ketamine/xylazine combination (100 mg/kg / 15 mg/kg) and their primary tumors were surgically excised. Each animal surgery wound was aseptically closed by one or two stitches. Ketoprofen, 1 mg/kg, was used 12 h before and after surgery for effective analgesia. Ophthalmic lubricant was used during the surgery to prevent corneal drying. Bupivicaine (0.25%, 1–2 drops), twice daily, was used topically at the excision wound site, local infiltration along the surgery site during closure with a maximum dose of 2 mg/kg. One day after surgery, mice maintained previous treatment groups: (i) the placebo control group (n = 5), (ii) the EF-2-treated group (n = 5). Treatments continued for an additional 30 days. The mice were routinely monitored by measuring tumor volume, body weight, and clinical observations. These observations included daily monitoring of general mice health characters (food/water intake, body weight, hydration status, defecation, urination, physical activity, and behavior). Wound incisions were carefully examined daily to ensure that wounds were contamination and inflammation-free, clean, and dry. All mice were then sacrificed, and individual tumors were excised, collected, and weighed. The results presented as average ± SD.
2.17. Western Blot Analysis
Collected breast tumor tissues after animal sacrifice were stored at −80 °C until protein extraction. To make sure every animal in each experimental group was represented in the western blotting results, equal parts of each collected breast tumor tissues were combined for each treatment group and homogenized in RIPA buffer (Qiagen Sciences Inc., Valencia, CA, USA) using an electric homogenizer. The protein concentration was then determined by the BCA assay (Bio-Rad Laboratories, Hercules, CA, USA). Equivalent amounts of protein were electrophoresed on SDS–polyacrylamide gels, whereas the gels were electroblotted onto PVDF membranes. Furthermore, these PVDF membranes were blocked with 2% BSA in 10 mM Tris-HCl containing 50 mM NaCl and 0.1% Tween-20, pH 7.4 (TBST), and then incubated with specific primary antibodies overnight at 4 °C according to the manufacturer protocol. At the end of the incubation period, membranes were washed five times with TBST and then incubated with respective horseradish peroxide-conjugated secondary antibody in 2% BSA in TBST for 1 h at room temperature, followed by rinsing with TBST five times. Blots were then visualized by chemiluminescence according to the manufacturer’s instructions (Pierce, Rockford, IL, USA). Proteins were detected using the ChemiDoc XRS chemiluminescent gel imaging system and analyzed using Image Lab software (Bio-Rad Laboratories). Here, visualization of β-tubulin was used to ensure equal sample loading in each lane. All experiments were repeated three times [
33].
2.18. Evaluation of EF-2 Treatment on the Human Serum CA 15-3 Biomarker Level
The cancer antigen 15-3 (CA 15-3) is a specific tumor marker usually used to monitor the response of the BC treatment and predict the potential of recurrence. Mice serum samples were collected at the end of the recurrence experiment and used for the determination of the concentration of the CA 15-3 without any modification. About 20 µL of serum sample was mixed with 1.0 mL of sample diluent, following the manufacturer protocol (Abnova, Catalog Number KA0206). About 200 µL of CA 15-3 standards, diluted specimens, and diluted controls were added into the appropriate wells and gently mixed for 10 s and incubated at 37 °C for 1 h, and the microtiter plate was rinsed and emptied 5 times by washing buffer (1×). Nearly 200 µL of enzyme conjugate reagent was dispensed into each well and gently mixed for 10 s followed by incubation at 37 °C for 1 h. In addition, 100 µL of TMB reagent was dispensed into each well and gently mixed for 10 s, followed by incubation at room temperature in the dark for 20 min. Finally, the reaction was stopped by adding 100 µL of stopping solution to each well and gently mixing for 30 s. Absorbance was read immediately at an optical density of 450 nm with a microtiter plate reader at the end of the experiment [
33].
2.19. Statistical Analysis
Values were expressed as mean ± standard deviation (SD) and differences among placebo and EF-2-treated groups followed by the analysis of unpaired t-test using GraphPad Prism version 8. Animal preference test was analyzed by One-way ANOVA followed by Tukey’s test. A difference of p < 0.05 was considered statistically significant as compared to the placebo control group.
4. Discussion
(–)-Oleocanthal is an exceptional natural phenolic compound exclusively occurring in EVOO with documented activities against cancer, inflammation, and Alzheimer’s disease [
1]. OC causes oropharynx irritative sensation via the activation of the transient receptor potential cation channel subfamily A, member 1 (TRPA1) receptor [
12,
13]. Despite its extensive biological studies, there is no OC systemic formulation reported so far to be used for subsequent preclinical and clinical applications. Thus, a taste-masked OC formulation is required to take this unique bioactive natural product to the next level, including future clinical assessments and therapeutic applications as a prospective nutraceutical. The oropharynx irritative taste of OC is not generalized acid-sensing and did not correlate with acid-induced irritation [
13]. OC and carbonic acid taste intensity ratings and mechanisms proved independent and were not correlated, which justify effective taste masking by the use of effervescent formulations [
13]. On the other hand, effervescent formulations have gained popularity in over–the–counter supplemental and pharmaceutical applications due to their easiness of use and favorable taste. The effervescent technique can also accelerate drug disintegration and dissolution, especially in quick-release preparations [
26].
This study optimized an OC effervescent powder formulation using various ratios of citric and tartaric acids and sodium bicarbonate. The dissolution of OC in water was significantly enhanced by reducing the solution pH due to the CO
2 release and formation of carbonic acid. The dissolution studies conducted at different physiologic pH mimicking the gastric and intestinal fluids suggested better palatability and improved absorption rate. The acidic pH of the solution was also important for improved formulation taste, whereas the acidic solution exhibits better palatability and good oral absorption [
26]. The measured pH of all formulations was below 6, which is considered an acceptable pH range [
26].
Favorable taste of oral formulation plays a prime role for ensuring patient acceptability, compliance, and commercial success. The taste assessment of EF-2 was essential to determine the success level for OC irritative taste masking. The e-tongue is an array of nonspecific, low-selectivity, chemical sensors with high stability and cross-sensitivity to different species in solution [
29,
30]. The e-tongue effectively discriminated the taste sensing of plain non-formulated OC on one side and the OC in EF-2 OC, and EF-2 placebo without OC on the other side. EF-2 and its placebo were not discriminated from each other as they nearly had similar DI values, indicating successful OC taste masking in EF-2. Effective taste masking was further confirmed by the animal preference test in which Swiss albino mice differentiated between plain non-formulated OC and EF-2, as the later was significantly favored and frequently consumed unlike the former, which had an unmasked irritative taste. Meanwhile, mice could not discriminate between EF-2 and its placebo formulation, as both had nearly similar preference.
Unlike its modest in vitro activity, extensive in vivo anticancer studies of OC have documented the remarkable tumor xenograft growth inhibitory potency not only against diverse molecular phenotypes of BC but also against a wide array of diverse tumors [
1,
3,
7]. Recent literature evidenced the superior OC activity against the luminal B BC represented by the ER
+/HER
+ BT-474 cells [
23]. This particular BC phenotype sensitivity is attributed to its overexpression of both OC validated molecular targets c-MET and ERα [
7,
19,
23]. This study confirmed the comparable in vivo potency of OC in EF-2 versus plain non-formulated OC against BT-474 BC progression using identical experiment conditions [
23,
33]. This study used an early treatment mode by starting EF-2 treatments immediately the second day after tumor cells xenografting based on earlier studies showing improved OC efficacy in this mode [
7,
23,
26]. It is, therefore, expected that OC in EF-2 was affecting the viability and colonization ability of engrafted tumor cells before solid tumor formation, which ultimately could contribute to the obtained delayed tumor growth. Recurrence of BC is one of the main mortality reasons among patient survivors who completed therapeutic interventions [
40]. To date, there are no formal recurrence inhibitors, as dormant tumor cells causing recurrence are usually resistant to existing targeted, chemo, and radio therapies [
33]. In addition, numerous studies suggested that different molecular subtypes of BC have a differential recurrence pattern [
41]. Clinical evidence demonstrates that TNBC is associated with higher five-year recurrence risk versus the 10-year recurrence range for the ER
+/HER2
+ BC [
40]. In this study, daily oral 10 mg/kg OC in EF-2 taken in adjuvant mode (post-surgical excision of the primary tumor) effectively inhibited BT-474 BC cells locoregional recurrence in nude mice xenograft after primary tumor surgical excision [
33]. It is expected that larger primary tumors are more likely to develop recurrence tumors after primary tumor resection compared to smaller tumors [
33,
42,
43,
44,
45,
46]. However, this study was designed to use the same animals to assess the OC in EF-2 suppressive efficacy for tumor growth and recurrence, mimicking clinical situations in which patients can use both neoadjuvant and adjuvant therapies or interventions before and after surgical tumor resection, respectively. Therefore, the final adjuvant mode suppressive effect of OC in EF-2 on tumor recurrence in this study also correlates well with its neoadjuvant mode suppression of the primary tumor. This finding further highlights the unique translational potential of EF-2 as a nutraceutical formulation of OC for long-term use to prevent luminal breast tumors recurrence supported by the safe food consumption of EVOO over human history.
The C-1 and C-3 aldehyde functionalities are associated with extensive unpredictable reactivity with endogenous amino acids, peptides, and proteins, and therefore, the pharmacokinetics of OC have never been identified. Extensive studies have validated the c-MET RTK and its downstream pathways including mTOR, Akt, STAT3, ERK1/2, MAPK, and HSP90 as OC in vivo anti-BC molecular targets [
7,
9,
18,
23,
26,
33,
39]. c-MET dysregulation can trigger aggressive proliferation and activation mechanisms of quiescent or dormant tumor cells, repopulation, and subsequent relapse and recurrence in breast and several other cancers [
42,
43,
44,
45,
46]. On the other hand, amplification of c-MET was also associated with the escape of cancer cells from the anticancer effects of several targeted therapies [
7,
26,
33,
43]. Therefore, this study relied on assessing the pharmacodynamics of OC in EF-2 by quantifying and comparing the activated and total c-MET levels in EF-2-treated and placebo-treated control tumor cell lysates. EF-2 treatment showed good suppression of p-c-MET levels in tumor progression mode and impressively reduced both the total and activated c-Met levels in recurrence tumors. Interestingly, EF-2 treatment significantly downregulated PI3K levels and suppressed the activation of p-AKT, p-STAT-3, and p-mTOR in recurrence tumors compared to the placebo group. Recurrence tumors are known to be more aggressive and invasive compared to their primary tumors, further confirming the recurrence suppressive potential of OC treatment [
33,
40,
41,
44]. Suppression of activated MET and downstream effectors in tumors further validated the pharmacodynamics efficacy of OC in EF-2.
It is well documented that each 2.6 adult mouse days are equivalent to one human year [
47]. After xenografting, treatment was started immediately to observe whether EF-2 exhibited any tumor progression delay. Here, EF-2 delayed the tumor progression onset by two days and tumor recurrence onset by four days. Therefore, OC treatment in EF-2 is predicted to delay luminal B breast tumor development and locoregional recurrence in humans by 9.2 and 18.5 months, respectively. In addition, the effect of EF-2 on the onset of tumor progression may be a reflection of the alteration of tumor engraftment or cell viability prior to engraftment as alternative mechanisms for obtaining the observation of the delayed growth curve.
The cancer antigen 15-3 (CA 15-3) is a protein produced by a variety of cells, especially BC cells and used as a tumor recurrence biomarker. CA 15-3 levels are higher than normal in most women with advanced metastatic BC [
33]. More than 96% of BC patients with local and systemic recurrence have elevated levels of CA 15-3 [
33]. EF-2-treated mice showed significantly reduced CA 15-3 sera levels compared to the placebo, indicating the potential of OC formulated in EF-2 as plausible BC recurrence inhibitor.