Imported: 13 Feb '17 | Published: 30 Jan '07
USPTO - Utility Patents
The invention relates to a process for detecting or determining a C-peptide-containing impurity in a sample of recombinantly produced human insulin or a derivative thereof, by a non-radioactive assay, comprising the steps:
Recombinant insulins are produced by down-stream processing of fusion proteins expressed by transformed E. coli. After the folding reaction, insulin precursors containing the correct disulfide bridges of insulin are cleaved out of the preproinsulin by treatment with trypsin, which liberates the C-peptide by synchronous cleavage at the sequence positions -Arg-Arg-(B31-B32) and -Lys-Arg-(A1-A0). The amino acid sequence of the monkey C-peptide differs from the human C-peptide by only one amino acid at position 37 (Pro vs. Leu).
After the purification process, the final product, human insulin, must be analyzed for the presence of minute amounts of preproinsulin and its derivatives, C-peptide containing insulin derivatives, and isolated C-peptide (collectively denoted as C-peptide-like activity). In the following, the abbreviations “HI” is used for “human insulin”, “HIA1” is used for “Gly(A21)-Arg(B31)-Arg(B32)-human insulin”, and “HIA2” is used for “Lys(B3),Glu(B29)-human insulin”, respectively. This analysis is usually performed by radioimmunoassay (“RIA”).
The disadvantage of the RIA method is that it requires a freshly iodinated tracer and a 3-day incubation period with all the connected unfavorable logistics. In addition, sample preparation is very time consuming, sensitive to changes, and previous measurements have shown that the assay, in some cases, is not free from unpredictable interference from tracer quality, sample handling, and precipitation during the 3-day incubation. The consequence is wasted time due to the need for repetitions. An additional drawback of the RIA method is that it cannot be applied on HIA1 or HI samples from purification steps before carboxypeptidase B (“CPB”) cleavage because these samples precipitate at the given pH of the assay method. Increasing the pH of the RIA assay buffer improves sample dissolution, but decreases the assay performance. The object of the present invention was to produce antibodies that can be applied in an immunoassay to quantify insulin C-peptide containing impurities in a final pure insulin preparation obtained in a specific purification batch (“end probes”) of HI (INSUMAN™), HIA1 (GLARGINE™), and HIA2 production, as well as in in-process batch step samples of the three insulin variants.
The antibodies should show affinity to isolated monkey C-peptide and preproinsulin (“PPI”), but they also should be able to bind to model compounds, such as PPI, human and/or monkey C-peptide, HI reduced/alkylated, HI cleaved with endoproteinase Asp-N at the EDP, HIA2 C-peptide, and HIA2 PPI, which are designed to reflect a panel of putative side products and impurities that can be anticipated in the industrial recombinant production of insulin.
An antibody preparation fulfilling the requirements described above can be used in a suitable assay format to quantify insulin C-peptide-like immunoreactivity in end probes of insulin purification, as well as in selected in-process batch step samples.
Due to the physical properties of the test batch step samples, the antibodies used must interact with the antigens with sufficient affinity at a pH of about 8.5–9.0. The interaction must not be influenced by the sample matrix, which is characterized by an insulin content of about 1 mg/mL. The immunoassay must be able to quantify C-peptide containing impurities (insulin C-peptide-like immunoreactivity) below 10 ng/mL (10 ppm compared to human insulin).
In theory, several C-peptide containing impurities can be expected as side products of insulin production. They are shown in FIG. 1A.
A process for detecting or determining a C-peptide impurity in a sample of recombinantly produced human insulin or a derivative thereof, by a non-radioactive assay, comprising the steps:
The present application describes a new non-radioactive immunoassay circumventing the above-described disadvantages of the original RIA. The new assay is based on:
Since the assay is performed at a pH=8.5−8.7, both HI and HIA1 test batch step samples remain dissolved during the whole incubation period and can be analyzed using the same assay without any variation.
Since the C-peptide of recombinant insulin HI and HIA1 is taken from monkey proinsulin, an assay suitable to detect monkey insulin C-peptide is needed to quantify insulin C-peptide-like immunoreactivity in HMR recombinant insulin end probes and in-process batch step samples.
The C-peptide of HIA2 is mutated and truncated at the C-terminal region when compared to the C-peptide of human insulin, hence it is artificial and not a naturally occurring peptide.
There are some diagnostic immunoassays available commercially that can be applied to determine the concentration of insulin C-peptide in serum. All assays show species specificity to human insulin C-peptide, some to rat, bovine, and porcine, but none to monkey C-peptide.
The purpose of the commercially available assays is to make them as specific as possible for either C-peptide or proinsulin. Our intention, however, was to obtain antibodies (an assay) that interact with both C-peptide and PPI with nearly the same affinity.
The antibodies used in the process of the present invention are obtained by immunizing a mammal, preferably a sheep, with the purified C-peptide of insulin, preferably the C-peptide of monkey insulin as described in European patent 0 032 675, followed by affinity purification with insulin immobilized on a suitable carrier, wherein the insulin preferably is semisynthetic human insulin or recombinant human PPI. The purification of the antibodies is described in the examples and attachments.
We tested three commercially available assays to determine whether they fulfill the needed requirements. The three assays were (1) RIA-coat C-peptide; (2) Human C-peptide RIA Kit; and (3) Human Proinsulin RIA Kit.
1. RIA-Coat C-Peptide (Cat. # 323.171 from BYK Sangtec Diagnostica GmbH & Co. KG 63120 Dietzenbach, Germany; 3-Hour Protocol)
This assay was not suitable to quantify monkey insulin C-peptide in the given samples. This may be due to:
This assay is based on antibodies against human C-peptide that show a 90% cross-reactivity to monkey C-peptide and <4% crossreactivity to human proinsulin.
The Linco assay can be applied to analyze the given HI samples. However, due to the pH of the kit buffers, precipitation of in-process batch step samples of HI production occurs. In the case of HIA1, which is not soluble at pH 7.4, the assay cannot be applied. A comparison with the chemiluminescent coated bead assay of the present invention shows that although the antibodies used in the assay are more potent in binding to isolated C-peptide from human and monkey origin, they do not interact as well with HI, PPI, or PPI cleaved at the EDP site as the antibody preparation of this invention. It should be noted that the coated bead assay of the present invention is performed at pH 8.7, whereas the Linco assay is performed at pH 7.4.
3. Human Proinsulin RIA Kit, Cat.# HPI-15K, K from Linco Research Inc. St. Louis, Mo. 63304-USA (3-Day Assay Protocol)
This assay is not able to detect monkey C-peptide, thus its specificity is not sufficient for our needs.
In conclusion, our requirements cannot be fulfilled by assays based on antibodies against human C-peptide or human proinsulin that are performed at neutral pH.
Requirements for the Assay are:
The invention will be described now by the examples, without being limited thereto.
In the following examples, some or all of the following materials and equipment were used:
Monosodium phosphate, Riedel (04269)
Sodium Chloride, Merck
Sodium azide, Merck
Serum albumin bovine, Behringwerke (ORHD 20/21)
Gamma globulin bovine, Sigma (G-5009)
Glycine, Riedel (33226)
Polyethylene Glycol-20% (“PEG-20”), Biodata
Sodium hydroxide-Fixanal, Riedel (38210)
Sodium hydroxide-2M, Riedel (35254)
Sodium acetate trihydrate, Merck (6267)
Phosphoric acid, 85%, Riedel (30417)
Semisynthetic human insulin (Hlet), Batch: A48 U114 and A48 U118
RIA Polystyrene tubes, 12×75 mm, Sarstedt (55476)
C-peptide second antibody beads, CPIII, Daiichi Radioisotope Labs; LTD, Tokyo
Proclin 300, Supelco Inc. (Bellafonte, Pa.) (4-8127)—a preservative for diagnostic reagents.
Dulbecco's Phosphate Buffered Saline (“PBS”), Sigma (D-5652)
Berilux solutions R1 and R2, Behringwerke (OCNH 02/03)—preformed analyzer reagents used to initiate chemiluminescence with acridinium labels. Solution R1 is composed of 0.5% H2O2 and 0.1M HNO3. Solution R2 is composed of a dilute solution of NaOH.
Sensor chips CM5, Pharmacia BIACORE®
Carbodiimide coupling kit, Pharmacia BIACORE®
BCA assay to quantitate protein concentration, Pierce (solution A: No.23223; solution B: 23224)
IgG Standard for determination of protein concentration, Pierce (31204)
Fractogel EMD Azlacton 650 (S), Merck (1.10087)
SDS Gel electrophoresis system using NuPAGE 4–12% and MES running buffer, Novex Silver Staining Kit Plus One, Pharmacia Biotech (Code # 17-1150-01)(All other Chemicals were purchased from Sigma, Merck, or Riedel de Haen.)
The antibodies were obtained by immunizing a sheep (S95-11) with purified insulin C-peptide. The serum sample obtained by bleeding the animal was purified according to the purification protocol in Example 1. The resulting affinity purified antibodies can be used in the present assay at a dilution of 1:1500.
The following antibodies were obtained:
Tracer and Standard Material:
Isolated C-peptide (monkey instead of human sequence) from recombinant human HI insulin is used as standard and tracer. The purity is 99%.
The sample buffer is prepared by dissolving 0.326 g Bicine (Sigma B-3876) in 100 mL distilled H2O. The pH is adjusted to 8.7 using 1N NaOH.
Semisynthetic human insulin (Hlet, see Materials) is dissolved in 10 mM HCl resulting in a 10 mg/mL solution. This insulin solution is further diluted 1:10 using sample buffer.
The commercially available PBS powder (Sigma D-5652) is dissolved in about 980 mL of water, the pH is adjusted to 7.7 by adding 1N NaOH. TWEEN-20® is added to yield a 0.05% final concentration of this detergent. The volume of the buffer is adjusted to exactly 1 L in a graduated cylinder. Finally, BSA is dissolved to yield 1.5% (w/v).
I. Assay Performance
A. Sample Dissolution:
About 0.5–0.8 mg of the sample was dissolved in 10 mM HCl to result in a 10 mg/mL solution. Subsequently, the completely dissolved material was further diluted 1:10 using sample buffer.
B. Tracer Preparation:
By using the acridiniumacylsulfonamide derivative Ki 256 (German patents DE 3805318 and DE 3628573, respectively), a C-peptide tracer can be produced by its direct covalent conjugation with a luminescent acridinium ester moiety. Chemiluminescence can be induced by addition of alkaline H2O2 (McCarpa, F. (1976) Acc. Chem. Res. 9: 201 ff.).
To prepare this chemiluminescent C-peptide tracer the following solutions were mixed and incubated for 20 minutes at ambient temperature:
The covalent reaction of the label with functional groups in the C-peptide was stopped by addition of 100 μl of L-lysine (10 mg/mL). The labeled C-peptide was purified by size exclusion gel-chromatography using a Superdex Peptide 10/30 column (Pharmacia) integrated in a FPLC system. The running buffer was PBS, 0.04% Proclin. Flow rate was 0.2 mL/min.
The protein eluted at the position between aprotinin and cytochrome C was collected, pooled, and frozen in aliquots (Batch 2). Reproducibility of tracer production has been proved once. The optimal dilution of the tracer stock solution for application in the assay was determined empirically.
C. Tracer Purification with Superdex-Peptide (10/30):
The elution of calibrators, cytochrome C(C), aprotinin (A), and vitamin B1 (B) are depicted in FIG. 2. Unreacted label, lysine, and other buffer components can be eliminated from the tracer that elutes in a symmetrical peak at a position between cytochrome C and aprotinin (bold line).
D. Standard Preparation:
A 1 mg/mL stock solution of insulin C-peptide (according to # 2 below) in water is prepared. From this stock an aliquot is taken and further diluted 1:100 in dilution buffer. Both samples are stored frozen until use.
The standards are prepared from the latter (10 μg/mL) sample by diluting an aliquot with dilution buffer 1:100 and subsequently 4-times by 1:4 resulting in standards of 100 ng/mL, 25 ng/mL (S4), 6.25 ng/mL (S3), 1.563 ng/mL (S2) and 0.39 ng/mL (S1). All dilutions are performed in dilution buffer containing 1 mg/mL Hlet in order to mimic the conditions in unknown insulin samples from production. Only standards S1–S4 are used in the assay.
II. Assay Protocol
A. Determination of Semisynthetic Human Insulin
The purpose of the assay is to identify PPI, insulin C-peptide, and putative insulin impurities that contain parts of the C-peptide covalently linked to the A- or B-chain of human insulin. These antigenic structures have to be determined in the presence of an excess of correctly processed insulin. To analyze the variation induced by this insulin background, semi-synthetic insulin (at concentrations of 1 mg/mL) was analyzed repeatedly. Semi-synthetic insulin must be used for this purpose to ensure that it does not contain impurities from monkey C-peptide, and in addition, it is a component of the standard/dilution buffer.
Semi-synthetic insulin was dissolved 10 fold at a concentration of 1 mg/mL as described in sample dissolution above. These insulin samples were analyzed in the assay on the same day. The results obtained with the insulin samples are shown in Table 1.
B0 represents the maximum response possible between the antibody and the tracer (without inhibitor) in a given test system (100% binding). B/B0 is the ratio of the signal obtained for a sample (standard or unknown) to B0. The results show that semisynthetic human insulin, on average, does not perturb binding of the tracer to the antibody. However, there is a certain variability resulting in maximal values of B/B0 of 106% and minimal values of 91%.
The consequence of this finding is that values above B/B0 of 90% (=mean value minus 3-times the standard deviation) obtained in unknown samples should not be calculated and was interpreted as background (detection limit). A sample yielding B/B0>90% most probably contains <0.4 ppm C-peptide-like activity. Repeatability (intra assay variability), as analyzed with these Hlet (background) samples, is 4%.
The same value of 4% was found in a sample that contains about 1 ng/mL C-peptide-like immunoreactivity (sample HIA1 055 AKR 01+02, see Table 1). Like the Hlet probe, the test batch step sample was dissolved 10-times at a concentration of 1 mg/mL and analyzed on the same day.
B. Applying the Statistical Approach on Insulin End Product Samples
The same statistical approach has been applied on end products of HI and HIA1 production. For this purpose, 23 different samples of HI and 6 different samples of HIA1 were analyzed on the same day. The results are shown in Table 2.
The data clearly show that end products of HI on average contain no C-peptide-like activity because the variation around the single determinations is similar to that obtained with semisynthetic human insulin. There is no significant difference between the HI group (of 24 individual samples) and HIet (one sample analyzed 10 times in the same assay). The minimum and maximum values in the HI samples (89.8% or 109%, respectively) also are close to the values obtained with HIet (91% or 106%, respectively).
Regarding HIA1 samples, it can be concluded that C-peptide-like immunoreactivity is present in the end product. However, the content is in the range of 0.5–1 ppm (when quantitated with the help of a standard curve, see below).
C. Standard Curve:
The standard curves obtained in 6 different assays are illustrated in FIG. 3. The numerical values of these curves expressed as B/B0 (%) are summarized in Table 3. The curve statistics are given in Table 4.
The curve parameters are:
The above presented data show:
The data presented above clearly show that when using the outlined method the assay is only minimally influenced by pure human insulin, which is the prerequisite for the determination of C-peptide-like immunoreactivity in pharmaceutical insulin preparations.
With the help of C-peptide containing model compounds, we investigated the specificity of our antibody in the given assay format. A description of the model compounds is given in the following sections.
The results of the control assays using the described model compounds as well as their interpretation is given in Example 7.
V. Summary Of The Control Experiments
The panel of model compounds excellently delineated the specificities of our affinity purified antiserum preparation 99Ser9.
In view of all the results shown, it can be concluded that the bead assay described in this application is well suited to measuring C-peptide-like immunoactivity in samples from insulin production. Because the antibody preparation in the given assay design recognizes all model compounds, it can be concluded that all major and putative C-peptide impurities in samples from recombinant insulin production most probably can be recognized by the assay.
Analysis of the Elimination of Contaminating C-Peptide Containing Impurities During Purification of HI Using the Described Assay of the Present Invention (Comparison with 3 Alternative Assays)
The new immunoassay has been applied to investigate the elimination of C-peptide-like immunoreactivity in 3 different production batches (UA114, UA0115, UA116). Test batch step samples from purification steps 12 (samples SB), 13 (samples KA and KB), and 14 (samples UA) have been analyzed for insulin C-peptide-like immunoreactivity.
The results obtained with the bead assay according to the present invention (“Beads”) are compared to the outcome of the commercially available alternative assays:
(1) HMR-RIA (“RIA”), (2) Human C-peptide RIA Kit from Linco Research Inc. (“Linco RIA”), and (3) the ELISA method developed by NewLab Diagnostic Systems GmbH (NewLab”).
Short Description of the Assays Used:
In the following tables, the following definitions of the abbreviations apply: SB: Test batch step samples after purification step 12 (ion chromatography); KA and KB: Test batch step samples after purification step 13 (RP-Chromatography); UA: Test batch step samples after purification step 14 (final crystallization); ND—not done.
The described new immunoassay of the present invention is very well suited for analyzing the elimination of C-peptide-like immunoreactivity in in-process control batch step samples from human insulin production with high sensitivity, precision, and good reproducibility (CV<5%).
The minimal concentration of insulin C-peptide-like activity that can be detected with the assay of the present invention is 0.4 ng/mL. Thus, all values below this value are considered background noise and should not be expressed numerically.
The Merits of the New Assay are:
Purification Of Sheep Anti Monkey Insulin C-Peptide Antibodies for Use in Immunoassays to Quantitate “Preproinsulin Like Activity”
Preparation Of Affinity Resins
I. Coupling of Human Insulin to Fractogel EMD Azlacton 650 (S)
A. Conditioning Of The Resin
7 g of Fractogel EMD Azlacton 650 (S) was allowed to swell for 15 minutes in 140 mL PBS, pH 7.4. After this incubation the supernatant was removed by passing through a glass filter. The remaining gel (about 24 mL) was suspended in 20 mL PBS, pH 7.4.
B. Dissolving Of Human Insulin
120 mg semisynthetic human insulin (insulin ET [insulin HPU, HGR, IE Sap. Nr.: 116312, Muster A48, Ch.-B.: U118]) was dissolved in 3 mL 50 mM phosphoric acid. The solution was slowly dropped into 50 mL PBS pH 9.4 with stirring. The pH dropped to 6.65 at the end of this procedure. Finally, the pH of the insulin solution was adjusted to 7.4 by careful addition of 2M NaOH.
C. Coupling Reaction
The insulin solution and the conditioned gel were mixed and the coupling reaction via the functional Azlacton was allowed to proceed at ambient temperature. A constant and careful mixing was achieved by slow head forward rotation of the reaction beaker. After 4 h at ambient temperature, the supernatant with unreacted ligand was filtered off and the gel was washed with 120 mL PBS. The HPLC analysis resulted in 67 mg of unbound human insulin. Consequently, 53 mg had been covalently immobilized on the EMD Fractogel.
Remaining active groups on the resin were blocked by addition of 120 mL 0.2 M glycine, pH 8.0. The reaction was allowed to proceed at ambient temperature. Again a constant and careful mixing was achieved by slow head forward rotation of the reaction beaker. After 16–20 h the supernatant with unreacted glycine was filtered off and the gel was washed with three cycles of 120 mL PBS pH 7.4, 0.1 M sodium acetate, 0.2 M glycine pH 2.8, each.
The resulting affinity resin with covalently coupled human insulin was suspended in PBS, pH 7.4, with 0.04% Proclin as preservative and poured into an HR 16/15 FPLC column.
II. Coupling of Human PPI to Fractogel EMD Azlacton 650 (S)
A. Conditioning of the Resin
14.5 g of Fractogel EMD Azlacton 650 (S) was allowed to swell for 15 minutes in 290 mL PBS, pH 7.4. After this incubation the supernatant was removed by passing through a glass filter. The remaining gel (about 50 mL) was suspended in 20 mL PBS, pH 7.4.
B. Dissolving of PPI
250 mg PPI was dissolved in 6 mL 50 mM phosphoric acid. The solution was slowly dropped into 100 mL PBS pH 9.4 with stirring. The pH dropped to 6.60 at the end of this procedure. Finally, the pH of the PPI solution was adjusted to 7.4 by careful addition of 2M NaOH.
C. Coupling Reaction
The PPI solution and the conditioned gel were mixed and the coupling reaction via the functional Azlacton was allowed to proceed at ambient temperature. A constant and careful mixing was achieved by slow head forward rotation of the reaction beaker. After 4 h at ambient temperature, the supernatant with unreacted ligand was filtered off and the gel was washed with 120 mL of PBS. The HPLC analysis resulted in 67 mg of unbound PPI. Consequently, 171 mg had been covalently immobilized on the EMD Fractogel.
Remaining active groups on the resin were blocked by addition of 250 mL 0.2 M glycine, pH 8.0. The reaction was allowed to proceed at ambient temperature. Again, a constant and careful mixing was achieved by slow head forward rotation of the reaction beaker. After 16–20 h the supernatant with unreacted glycine was filtered off and the gel was washed with three cycles of 250 mL PBS pH 7.4, 0.1 M sodium acetate, 0.2 M glycine pH 2.8, each.
The resulting affinity resin with covalently coupled PPI was suspended in PBS pH 7.4 with 0.04% Proclin as preservative and poured into a XK 26/20 FPLC column.
III. Purification of Anti Insulin C-Peptide Antibodies By Affinity Chromatography
A. Preparation of the Serum
The source for antibody purification was a serum from sheep S95-11, which was immunized with purified monkey insulin C-peptide. The serum had been stored at −20° C. until thawing. After thawing, the total volume of the serum was determined to be 79 mL. The volume was doubled by adding 65 mL water and 16 mL PBS (10× concentrated, plus 0.4% Proclin) to adjust the buffer conditions for chromatography. The diluted serum was subsequently centrifuged at 26,000×g for 30 minutes at 4° C. The supernatant was filtered through stacked 5 μm and 0.2 μm membranes protected by a glass filter layer.
B. Affinity Chromatography
The rationale for the purification scheme was to remove antibodies that nonspecifically bind to the EMD Fractogel resin and sequences in human insulin as well as putative sheep anti-insulin antibodies in a first step by passing the serum through the human insulin affinity resin.
In a second step anti-monkey insulin C-peptide antibodies can be purified after their binding to the C-peptide, which is an integral part of PPI immobilized on EMD Fractogel.
Affinity chromatography on PPI was chosen (instead of chromatography on immobilized C-peptide), because in order to use the purified antibodies in an immunoassay for quantification of PPI-like immunoreactivity, binding to C-peptide and/or C-peptide fragments still connected to the insulin moiety is a prerequisite.
IV. Purification Protocol
A. Sample Application
The diluted sheep serum was pumped through the two affinity columns (flow: 3.5 mL/min) in one step by directly connecting the human insulin EMD Fractogel HR16/11 column (“first column”) to the PPI EMD Fractogel XK26/11 column (“second column”). The first column eliminates all undesired binders, but does not interact with anti-insulin C-peptide antibodies. The second column specifically binds anti-monkey insulin C-peptide antibodies.
Nonspecific sheep antibodies, as well as serum components, cannot interact with the affinity resin of the second column and thus, flow through the column into the eluate. Total capture of specific antibodies by the affinity column was checked by analysis of the flow-through making use of the BIACORE® system. In the flow-through fraction, there was no binding activity detectable.
After passage of the serum, the two columns were disconnected and independently eluted with 0.1 M glycine, 0.04% Proclin pH 2.7 (flow: 5 mL/min).
In the case of the first column, the acidic elution directly results in regeneration of the affinity matrix, which can subsequently be conditioned by extensive equilibration with PBS buffer.
FIG. 4A depicts the elution profile of a human insulin chromatographed on an Azlacton HR 16/11 column. The elution profile of the PPI EMD Azlacton column is shown in FIG. 4B. Fractions 9–22 contain active anti-monkey insulin C-peptide antibodies as analyzed with the BIACORE® system. The indicated fractions were pooled and concentrated to 10 mL using Amicon Ultrafree-15 units (10 kDa molecular weight cut off membranes).
To further purify the antibodies and to transfer them into a neutral buffer, the concentrated glycine eluate was chromatographed on a Superdex 200 (26/60) size exclusion column. The elution profile is shown in FIG. 5. Fractions 17–24 contain the purified sheep anti monkey insulin C-peptide antibodies. To achieve higher purity of the antibodies, a second gel filtration chromatography was performed using the same Superdex 200 (26/60) column (FIG. 6). The protein eluting in fractions 17–21 was pooled and stored as the final antibody preparation “99Ser9-rechr”.
The flow-through of the affinity tandem column as well as the eluants of the PPI EMD Fractogel and Superdex 200 columns were analyzed using the BIACORE® system. This technique allows the fast detection of anti-monkey insulin C-peptide antibodies by simulation of the affinity chromatography in a 60 nL flow cell generated on the surface of a sensor chip. Active antibodies binding to PPI immobilized in this flow cell can be detected by surface plasmon resonance with high sensitivity.
The volume of the antibody preparation was 24 mL as determined with a graduated cylinder. Protein concentration was determined by the BCA method in a micro well format according to the instructions of the supplier (Pierce). The OD at 560 nm was measured using a Spectra III Elisa reader (SLT).
A standard curve with IgG of known concentrations was used to calculate the unknown concentration of the antibody preparation. The mg/mL value of the unknown was read directly from the plotted data.
The described antibody preparation had a concentration of 0.454 mg/mL. The total yield of antibody was 10.9 mg. The antibody preparation was aliquoted in 1 mL portions, each labeled with 99Ser 9/rechr.F17-22, and stored at −70° C.
The above described antibody preparation (99Ser 9/rechr.F17-22) can be used in immunoassays to quantitate insulin C-peptide-like immunoreactivity, especially in the assay variant coated bead chemiluminescence assay.
Alkylation of Human PPI (Batch 216-1):
3.28 mg of PPI was dissolved in 109 μL H2O and further diluted by adding 109 μl 10-fold concentrated PBS buffer supplemented with 0.4% Proclin. Finally, a solution was prepared with a PPI content of 3 mg/mL by adding 875 μL H2O. To 990 μl of this PPI solution, 10 μL of 1M DTE (in water) was pipetted. The sample was incubated at 37° C. for 5 hours. After 1 h reaction time, precipitation of protein was detectable and still present after raising the pH to 9.0 for the remaining time.
To stop the reduction of S—S bridges in PPI and to protect free sulfhydryl from reoxidation and/or generation of new S—S bonds, 185 mg of solid iodoacetamide was added and incubated 4 hours in the dark at 4° C.
The resulting alkylated PPI was then dialyzed twice against 400 mL PBS, 0.04% Proclin in a Tube-O-Dialyser. The precipitated protein was removed by centrifugation. In the clear supernatant (1.3 mL), 23 μg/mL of soluble alkylated PPI could be determined by amino acid analysis after hydrolysis of the sample.
A slightly retarded penetration in SDS-gel electrophoresis amino acid analysis and a better susceptibility to proteolytic degradation with endoproteinase Asp-N (See Example 6) proved the effective derivatization. Obviously, endoproteinase Asp-N cleaves the reduced PPI at the derivatized cysteines, in addition to the DP-site.
Cleavage of Human PPI at the EDP site with Endoproteinase Asp-N
Endoproteinase Asp-N is a metallo-protease that specifically cleaves peptide bonds N-terminally at aspartic and cysteic acid. Human PPI (HIA1 PPI) contains only one aspartic acid in its sequence. It is located in the C-peptide where it is situated N-terminally to a proline residue, creating the acid labile DP (C6–C7) site. This site presumably is located on the outer surface of the PPI molecule and should therefore be accessible to endoproteinase Asp-N. The cysteine residues all are involved in S—S bridges, buried within the molecule and therefore protected from proteolytic attack. By splitting PPI at the EDP site, a valuable model compound can be generated helping in delineating the specificity of our affinity purified antibodies.
To cleave the EDP site within the C-peptide 50 μl of PPI (0.7 mg/mL in water) and 50 μl of 50 mM Tris/HCl pH 8.0 containing 0.2 M urea were mixed. Proteolysis was started by the addition of 25 μl endoproteinase Asp-N (1 μg dissolved in 10 mM Tris-HCl, pH7.5) and was incubated for 32 hours at 37° C. The reaction was stopped by freezing the sample.
Gel electrophoresis and subsequent silver staining showed that PPI was quantitatively cleaved at the EDP site because two bands can be separated after reduction of the disulfide bonds in the enzymatically cleaved protein (See Example 6). There is no remaining protein band visible at the position of uncleaved reduced PPI (which still is a single chain molecule).
The endoproteinase Asp-N cleaved PPI can serve as a model (control) compound that:
Analysis of HI and HI Reduced/Alkylated, as Well as the Endoproteinase Asp-N Cleaved Products Thereof with SDS Gel Electrophoresis (4–12%)
The samples were applied to an electrophoresis gel in SDS-buffer containing DTE to completely reduce the S—S bridges. See FIG. 7.
Specificity of the Affinity Purified Antibodies From Sheep S95-11 in the Described Assay Format
In the original RIA (1) and in the Linco assay (2) (See Example 2 above), the concentrations have to be on the order ≧20, ng/mL. It is not possible to give respective data for the ELISA developed by NewLab (3), because the controls were not performed and there is no data about the detection limit of PPI using this method. Deduced from the assay design, it can be anticipated that the ELISA would show lower levels of cross reactivity to PPI and PPI-derivatives than the Linco assay.
Summarizing all the results obtained with the model compounds it clearly can be stated that the antibodies obtained from sheep S95-11 preparation 99Ser9 (and 99Ser7 which is used in the RIA) fulfill the requirements to positively identify different kinds of C-peptide containing antigens, which can be circumscribed with “C-peptide-like immunoreactivity”.
Analysis Of In-Process Batch Step Samples From Insulin HIA1 Production—Elimination of Insulin Like Immunoreactivity During Down Stream Processing.
Immunization and Sampling of Sera From a Sheep (S95/11) with Polyclonal Antibodies Directed Against Monkey C-Peptide
This example describes and documents the immunization and sampling of sera from a sheep, containing antibodies directed against C-peptide from monkey.
A female sheep was purchased from Gerhard Mundschenk (Zwerggasse 2; 65468 Trebur-Astheim) and maintained on normal standard diet for goat on a farm (Hermann Kettenbach, Im Birkenfeld 38, 65719 Hofheim-Langenhain). The sheep was marked S95/11.
At the beginning of immunization, the sheep (marked S95/11) was immunized with antigen (initially 2 mg monkey C-peptide) in equivalent volumes (1:1 mixture of 1 mL saline +1 mL complete Freund' adjuvant (cFA; Difco Laboratories, Detroit, Mich., USA)). This emulsion was prepared immediately prior to administration and was injected subcutaneously at 2–3 sites. Booster injections at 2–4 week intervals consisted of the same amount of antigen (2 mg C-peptide) in a 1:1 mixture of saline (1 mL) and 1 mL of incomplete Freund's adjuvant (Sigma Chemicals, Heidelberg, Germany). At intervals of 2 weeks to 2 months, blood samples were taken by puncture of jugular vein. The antiserum was aliquoted and stored at −20° C. until further use. Specified samples were selected for development of an assay for detection of PPI in the final insulin product.
Recombinant human insulin is produced from a fusion protein expressed in transfected E. coli. During the processing of human insulin from the denatured fusion protein the so-called PPI is formed as an intermediate. The latter is further processed by enzymatic trypsin-catalyzed cleavage. The two chain heterodimeric insulin is formed from the single chain PPI by synchronous cleavage at the sequence positions -Arg-Arg-(B31-32) and -Lys-Arg-(A-1-A0). Insulin is produced via further purification processes. For development of an immunoassay for detection of impurities of PPI (<10 ppm) in the final product polyclonal antibodies directed versus C-peptide are particularly suited because there is no interfering cross-reactivity from insulin. In contrast, in the case of immunization with PPI as antigen, the major amount of antibodies would be directed against insulin. The latter would not be suited to detect minute amounts of <10 ppm PPI in the presence of a 106 fold excess of insulin.
To avoid immunogenic insulin-like determinants in the C-peptide used, like -Lys or -Lys-Arg- at position 34 or 34–35 of the C-peptide, monkey C-peptide without these basic amino acids Arg- has been prepared and used as monkey C-peptide.
The immunization procedure including the switch from initial use of cFA to iCA and time scheme corresponds to standard methods as described in the literature for production of polyclonal antibodies directed versus specified antigens and peptides.
Documentation of the immunization scheme and blood sampling for preparation of antisera with polyclonal antibodies against monkey C-peptide in a sheep are presented. The amount of antigen (monkey C-peptide) was dissolved in a 1:1 mixture of 1.0 mL 0.9% saline and 1 mL adjuvant, respectively. Dates of blood sampling are listed. The antigen used in the following study was monkey C-peptide; Lot no.:DJIII,-S43. The initial adjuvant used was KFA (Difco) Lot 70052LA. The adjuvant Booster was IFA (Sigma) Lot 96H8950.
It is known that insulin is relatively insoluble in neutral solutions after crystallization or freeze-drying. In order to obtain high concentrated insulin solutions, insulin probes therefore must first be dissolved in acid (e.g., diluted phosphorous acid or HC, either) and afterwards an alkaline pH (9.0–10.5) has to be adjusted by fast addition of the appropriate amount of NaOH. The working pH is then adjusted by titration or by adding respective buffers. This outlined procedure is very time consuming, fussy, and requires individual care for each probe.
To circumvent this ceremonious procedure we tested different protocols for insulin dissolution. The aim was to obtain clear insulin solutions with a concentration of 1 mg/mL and a pH of 8.6–9.0.
We tested different buffers suitable in the pH range 8.0–9.5 in order to screen for stable antigen (HI PPI)/antibody binding at pH 9.0. Buffers tested were: Tris, TAPS, Bicine, GlyGly, BisTrisPropane, Ches, Phosphate/EDTA (as used in the human C-peptide RIA Kit from Linco Research Inc.).
In addition, different buffer concentrations have been analyzed showing that increasing the buffer concentrations at the given pH 9.0 results in a gradual disturbance of antigen/antibody binding.
In view of the previous discussion, we selected four buffers at a concentration of 20 mM for final comparison:
1. Tris* (Merck-108382),
2. TAPS** (Sigma T-5130),
3. Bicine*** (Sigma B-3876)
4. GlyGly**** (Calbiochem 3630).
We prepared these buffers by dissolving the needed amount of solid in water and adjusted the pH to 9.0 by adding the appropriate amount of NaOH. All show good antigen/antibody binding in the pH-range of about 8.5 to about 9.0.
About 0.5 mg–0.8 mg of insulin samples were dissolved in 10 mM HCl resulting in a 10 mg/mL solution. Subsequently, the clear dissolved material was further diluted 1:10 using one of the above buffers. The outcome is shown in the following Table:
In all examples the pH is unaltered or only minimally influenced after the addition of the acidic insulin solution. However, to our surprise only in Bicine buffer could a clear solution of HIA1 or in-process batch step samples be obtained consistently.
Therefore, choosing the Bicine buffer system represented a major break through in establishing a simple sample dissolution procedure and in obtaining stable analyte/buffer conditions during the incubation period of the immunoassay.