Development of an enzyme immunoassay for a stable amidated analog of the hemoregulatory peptide acetyl-Ser-Asp-Lys-Pro

An analytical quality by design approach towards a simple and novel HPLC-UV method for quantification of the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline

N-acetyl-seryl-aspartyl-lysyl proline (Ac-SDKP) is a tetrapeptide possessing anti-fibrotic, angiogenic, anti-inflammatory, anti-apoptotic, and immunomodulatory properties. Currently, the main method to quantify the peptide is liquid chromatography-tandem mass spectrometry (LC-MS/MS) and enzyme-linked immunosorbent assay (ELISA), both of which are labour intensive and require expensive equipment and consumables. Furthermore, these techniques are generally utilised to detect very low or trace concentrations, such as in biological samples. The use of high concentrations of analyte might overload the extraction column or the separation column in LC-MS/MS or the ELISA plates, so the response could be a non-linear relationship at high analyte concentrations. Thus, they are not ideal for formulation development where detection of dose-equivalent concentrations is typically required. Therefore, a cost-effective, simple, and accurate quantification method for the peptide at a higher concentration needs to be developed. In this study, a simple and novel HPLC-UV method is proposed and validated using an Analytical Quality by Design (AQbD) approach. The method is first screened and optimised using chromatographic responses including capacity factor, resolution, tailing factor, and theoretical plate counts, fulfilling the International Council for Harmonisation (ICH) Q2 (R1) guidelines. The resultant optimised chromatography conditions utilised 10 mM phosphate buffer at pH 2.5 and acetonitrile as mobile phases, starting at 3% (v/v) acetonitrile and 97% (v/v) buffer and increasing to 9.7% (v/v) acetonitrile and 90.3% (v/v) buffer over 15 minutes at a flow rate of 1 mL/min at the column temperature of 25 °C. The injection volume is set at 10 μL and the VWD detector wavelength is 220 nm. The method established is suitable for detecting the peptide at a relatively high concentration, with a quantifiable range from 7.8 μg/mL to 2.0 mg/mL. In addition, the use of a relatively simple HPLC-UV approach could significantly reduce costs and allow easier access to quantify the peptide concentration. A limitation of this method is lower sensitivity compared with using LC-MS/MS and ELISA methods but running costs are lower and the methodology is simpler. The method is capable to quantify the peptide in various tested matrix solutions, with successful quantitation of the peptide in samples obtained from in vitro drug release study in PBS and from a chitosan-TPP nanogels formulation. Therefore, the method developed here offers a complementary approach to the existing quantification methods, quantifying this peptide at increased concentrations in simple to intermediately complex matrix solutions, such as HBSS, DMEM and FluoroBrite cell culture media.

Two Opposing Functions of Angiotensin-Converting Enzyme (ACE) That Links Hypertension, Dementia, and Aging

A 2018 report from the American Heart Association shows that over 103 million American adults have hypertension. The angiotensin-converting enzyme (ACE) (EC 3.4.15.1) is a dipeptidyl carboxylase that, when inhibited, can reduce blood pressure through the renin–angiotensin system. ACE inhibitors are used as a first-line medication to be prescribed to treat hypertension, chronic kidney disease, and heart failure, among others. It has been suggested that ACE inhibitors can alleviate the symptoms in mouse models. Despite the benefits of ACE inhibitors, previous studies also have suggested that genetic variants of the ACE gene are risk factors for Alzheimer’s disease (AD) and other neurological diseases, while other variants are associated with reduced risk of AD. In mice, ACE overexpression in the brain reduces symptoms of the AD model systems. Thus, we find two opposing effects of ACE on health. To clarify the effects, we dissect the functions of ACE as follows: (1) angiotensin-converting enzyme that hydrolyzes angiotensin I to make angiotensin II in the renin–angiotensin system; (2) amyloid-degrading enzyme that hydrolyzes beta-amyloid, reducing amyloid toxicity. The efficacy of the ACE inhibitors is well established in humans, while the knowledge specific to AD remains to be open for further research. We provide an overview of ACE and inhibitors that link a wide variety of age-related comorbidities from hypertension to AD to aging. ACE also serves as an example of the middle-life crisis theory that assumes deleterious events during midlife, leading to age-related later events.

Synthesis and bioactivity of a Goralatide analog with antileukemic activity

Natural tetrapeptide Goralatide (AcSDKP) is a selective inhibitor of primitive haematopoietic cell proliferation. It is not stable in vivo and decomposes within 4.5min when applied to live cells. In this work we developed an analog of Goralatide that exhibits cytotoxicity towards human myeloid HL-60, HEL, Nalm-6 leukemia cells, endothelial HUVEC, glioblastoma U251 and transformed kidney 293T cells. The Goralatide analog showed significant stability in organic solution with no tendency to degrade oxidatively.

Synthesis of L-Lys-Aminoxy-Goralatide

Natural tetrapeptide Goralatide inhibits primitive hematopoietic cell proliferation but reported to be rather unstable in solution (half-life 4.5 min). In this work, we report the synthesis of an aminoxy analog of Goralatide. Aminoxy moiety is expected to provide increased stability and bioavailability of the Goralatide analog.

Mass spectrometric quantification of AcSDKP-NH2 in human plasma and urine and comparison with an immunoassay

RATIONALE

Precise assessment of renal glomerular filtration rate (GFR) is essential for the early detection of chronic kidney disease. AcSDKP-NH(2), an analogue of the endogenous tetrapeptide AcSDKP, is not degraded in vivo and is freely filtered by the kidney and eliminated in urine; for that reason this analogue is an ideal candidate marker for the assessment of GRF after administration to humans. Proof-of-concept demonstration and lack of toxicity in animals have allowed an ongoing clinical study in which AcSDKP-NH(2) was administered intravenously at a dose of 100 µg and compared with currently available GFR markers. The use of the AcSDKP analogue in clinical practice requires that this novel marker be associated with an analytical method that combines specificity, robustness and high accuracy. We have developed a liquid chromatography/tandem mass spectrometry (LC/MS/MS) assay and compared it with an existing enzyme immunoassay (EIA) for AcSDKP-NH(2).

METHODS

Human urine and plasma samples from the clinical study were analyzed by EIA and LC/MS/MS. Before LC/MS/MS assessment, AcSDKP-NH(2) was extracted using mixed-mode cation-exchange solid-phase extraction cartridges. Chromatographic separation was performed by hydrophilic interaction liquid chromatography (HILIC), before analysis with an electrospray ionization triple quadrupole mass spectrometer.

RESULTS

Mass spectrometry, through the use of an internal standard, tailored sample preparation and chromatographic separation, has better intra- and inter-assay precision (accuracies between 95 and 101% with CVs <8% for LC/MS/MS vs. accuracies between 90 and 115% with CVs <18% for EIA) and allows greater steadiness in intra-subject concentrations during the infusion (4.4% for LC/MS/MS vs. 8.6% for EIA). Moreover, the LC/MS/MS assay circumvents matrix effects observed in certain instances for the EIA and which may reduce its accuracy.

CONCLUSIONS

Although the EIA can provide sufficient information in most subjects, the LC/MS/MS assay associated with this new marker should be the reference method.

On-line solid-phase extraction LC-MS/MS for the determination of Ac-SDKP peptide in human plasma from hemodialysis patients

We developed a high-throughput method based on on-line solid-phase extraction liquid chromatography tandem mass spectrometry (SPE-LC-MS/MS) to determine N-terminal thymosin-β fragment peptide (N-acetyl-seryl-aspartyl-lysyl-proline, Ac-SDKP) in human plasma samples. Quantification of Ac-SDKP was performed using direct injection for on-line SPE based on C(18), reversed-phase LC separation and stable isotope dilution electrospray ionization-MS/MS in multiple reaction-monitoring (MRM) mode. The Ac-SDKP-(13)C(6), (15)N(2) (m/z 496 → 137) was synthesized for the internal standard. The MRM ion for Ac-SDKP was m/z 488 → 129 (quantitative ion)/226. The limit of detection and lower limit of quantitation were 0.05 and 0.1 ng/mL in standard solution, respectively. Recovery values were 98.3-100.4% with inter-day (relative standard deviation, RSD, 0.4-14.1%) and intra-day (RSD, 0.8-19.7%) assays. This method was applied to the measurement of Ac-SDKP levels in plasma from hemodialyzed subjects. Concentrations were 0.59 ± 0.23 ng/mL (pre-hemodialyzed subjects, n = 9) and 0.44 ± 0.19 ng/mL (post-hemodialyzed subjects, n = 9). All plasma Ac-SDKP levels were decreased by dialysis. Thus, plasma Ac-SDKP was decreased through dialysis in chronic kidney disease. The findings in this study will be useful for the treatment of anemia in chronic kidney disease with dialysis.

Quantification of N-acetyl-seryl-aspartyl-lysyl-proline in hemodialysis patients administered angiotensin-converting enzyme inhibitors by stable isotope dilution liquid chromatography-tandem mass spectrometry

We developed a sensitive, selective and accurate method based on liquid chromatography with tandem mass spectrometry (LC-MS/MS) to determine N-terminal thymosin-β peptides of Ac-SDKP and Ac-ADKP in human plasma samples. Quantification of Ac-SDKP and Ac-ADKP was performed using solid phase extraction (SPE) based on C(18), reversed phase LC separation, and stable isotope dilution electrospray ionization-MS/MS in multiple reaction-monitoring (MRM) mode. The Ac-SDKP-(13)C(6), (15)N(2) and Ac-ADKP-d(7) were synthesized for the internal standards. These MRM monitoring ions were m/z 488→129 (quantitative ion)/226 for Ac-SDKP, m/z 496→137 for Ac-SDKP-(13)C(6), (15)N(2), m/z 472→129 (quantitative ion)/226 for Ac-ADKP, and m/z 479→129 for Ac-ADKP-d(7), respectively. Lower limit of quantitation (LLOQ) of Ac-SDKP and Ac-ADKP was 0.1ng/mL in human plasma. Recovery values were ranged from 94.7% to 106.3% for inter- (RSD: 0.6-3.5%) and intra- (RSD: 0.4-4.9%) day assays. Plasma Ac-SDKP levels were significantly higher in hemodialyzed subjects treated with angiotensin-converting enzyme inhibitors of enalapril (27.3±24.6ng/mL, n=10) and trandolapril (12.3±16.9ng/mL, n=18) than healthy (0.4±0.2ng/mL, n=7) and hemodialyzed subjects (0.6±0.2ng/mL, n=34). This analytical method would be useful to measure N-terminal thymosin-β peptides in human plasma for the clinical study.