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7.9 Sample Stability Considerations 183
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500 copies/reaction. This spike‐in approach is particularly useful for liquid speci­mens, such as urine, saliva, blood, plasma, and semen, which are monitored in clinical vector shedding studies. While feasible, this approach faces obvious diffi­culties with solid samples. These difficulties may be overcome by preparing mul­tiple specimen portions to accommodate single extractions at each stability time point, and by adapting testing procedures to lyse stability specimens directly in their original containers.
To assess stability concurrently to or after a study, biological specimens from GTx‐treated individuals can be extracted and tested using a primary PCR test that serves as baseline assessment, followed by multiple secondary extractions and PCR retests of the same specimen (or aliquots/portions thereof) at defined stabil­ity intervals. This would not require spiking any vector material and resembles incurred sample reanalysis (ISR), which is sometimes performed for validation of other pharmacokinetic methods. This retest approach is useful for tissue speci­men in biodistribution studies, where spiked surrogate vector DNA or intact GTx vector capsids may not fully recapitulate the biological conditions that affect epi­somal vector DNA within the nuclei of transduced cells. In particular, linearized surrogate vector DNA spiked onto tissue specimens may represent more of a worst‐case scenario for analyte stability, since circularly closed vector episomes within cells would be expected to be more stable than open‐ended surrogates that could be exposed to extracellular DNases in tissue samples. The obvious disadvan­tage of using secondary PCR tests of incurred specimens for stability studies is that the results cannot inform study design, and thus sample storage conditions and duration are decided at risk. Another consideration is limited sample quantity in small animal studies and the need to plan for collecting multiple specimen aliquots/portions to facilitate evaluation after various storage intervals without incurring freeze–thaws.
As for extracted DNA stability, most PCR workflows aim to test shortly after DNA samples have been extracted from biological specimens. In addition, the sta­bility of purified DNA frozen at low temperature is relatively well established, and thus long‐term stability studies may not be needed. Considering the possibility of PCR retests following a failed initial run, freeze–thaw assessments combined with short‐term stability periods (e.g. 1month) appear to be more useful. This evalua­tion can be performed pre‐study using spiked surrogate vector DNA, or post‐study by repeated PCR testing of extracted DNA samples after the primary test was com­pleted on study.
In‐assay short‐term stability assessments differ between qPCR and ddPCR. In qPCR, one would typically proceed from sample preparation to amplification and generate results in real time. Using Bio‐Rad’s QX200 ddPCR system, it may be pos­sible to “pause” the run for a period of time following thermal‐cycling of the gen­erated droplets (e.g. overnight). This allows for greater analyst flexibility and
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increased throughput, since test plates can be prepared and thermal‐cycled on Day 1 and read out on Day 2, with runs staggered by one day. Furthermore, hold­ing droplets at 12 more droplets will meet the instrument’s droplet quality metrics [24]. A higher number of acceptable droplets translates into a higher probability that low abun­dance targets will be detected.
Stability acceptance criteria for PCR‐based methods vary. Measured GTx vector quantities in stability samples can be compared back to baseline samples to reveal any trends of degrading sample integrity. Alternatively, a comparison to nominal spike levels can be made. The latter approach lends itself more easily to extracted DNA samples spiked with known amounts of vector, while the former approach is more applicable for biological specimens, where extraction variability and unknown vector quantities preclude the use of nominal concentrations.
When comparing results from stability time points to those from baseline sam­ples, a threshold needs to be established for the biological specimen to be consid­ered stable. In other words, what relative difference from baseline results indicates sample instability? No consensus exists but reasonable acceptance ranges may fall between 0.5 and 0.2 times baseline value. To justify a threshold, the variability of both DNA extraction efficiency and PCR measurement needs to be considered. For example, if extraction efficiency varies by a factor of 2, then halving of PCR‐ measured vector quantities would not be conclusive for specimen instability. Assuming a potential negative bias of −50% for both extraction efficiency and PCR measurements, one would obtain a theoretical lower limit for overall analyti­cal variability of 0.5 × 0.5 = 0.25. Thus, after adding a small margin of tolerance, a value of 0.2 times baseline could be a reasonable threshold, below which biologi­cal specimens are considered instable.
°C for a minimum of 4 hours can increase assay sensitivity, since
7.10   Data Reporting Formats, Acceptance Criteria, 
and Trending
Depending on study context, PCR data may be reported in various formats: In nonclinical biodistribution studies, VG copies per microgram of gDNA are com­monly used, and VG copies per mg tissue may also be reported. The former unit can be converted to VG copies per cell by multiplying with the approximate mass of a diploid genome (6 pg in humans/mice, 5 pg in NHP). Potentially higher degrees of ploidy, for example in liver and heart cells[25–27], may need to be considered as appropriate. In shedding studies, reporting as VG copies per mL biological fluid or mg solid specimen is oftentimes more clinically relevant, since volumes and masses are more easily visualized. For clinical specimens with suf­ficient DNA content, such as whole blood and seminal fluid, reporting as VG cop­ies per microgram gDNA could additionally be provided.
7.10 Data Reporting Formats, Acceptance Criteria, and Trending 185
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Exemplary acceptance criteria for regulated PCR sample testing are provided in Tables7.3 and7.4. The standard curve and low‐level quality control (QC) within 1log of the LLOQ are routinely included, together with various negative controls that monitor for potential cross‐contamination. Medium and high QCs can be added to increase confidence in accurate and precise sample quantification.
The range of acceptable accuracy for qPCR‐based methods is typically broader (−50% to +100% relative error, RE) than that of other pharmacokinetic methods, as discussed above. When performing qPCR, it can be useful to verify that no sample‐ mediated inhibition occurred, in particular for samples without detectable increase in fluorescence that would be reported as negative. Sample‐mediated inhibition can be assessed by spiking a low quantity of vector DNA (50–500
copies/reaction) into a designated test sample replicate and comparing with a spike volume control (SVC) sample. The SVC sample contains the same number of vector DNA
Table7.3  Exemplary standard curve, QC, and run acceptance criteria forPCR.
Acceptance criteria qPCR sample 
Parameter
Standard Curve Regression
Precision The CV of replicate Cq or Ct values
Accuracy The RE for each standard
Negative controls
analysis
The standard curve should have an
2
R
≥ 0.98 and a slope between −3.1 and −3.6 (corresponding to 90–110% amplification efficiency).
for each calibration point and quality control (QC) sample should be ≤3%. OR: The CV of interpolated VG copies for each standard curve point and QC samples should be ≤30%, except for LLOQ‐QC where CV should be ≤50%. Outlier exclusion: 1 out of 3 replicates may be omitted, if the Cq or Ct differences between the omitted replicate and each of the other two replicates are greater than 1.
calibration point and QC sample should be within −50% and +100% of the nominal value.
Negative extraction controls, no‐template controls, and sentinel controls should not show any increase in fluorescence OR: remain below the LOD.
Acceptance criteria dPCR sample 
analysis
Not applicable, absolute quantification mode alleviates the need for a standard curve
The CV of measured VG copies for QC samples should be ≤30%, except for LLOQ‐QC where CV should be ≤50%. Outlier exclusion: 1 out of 3 replicates may be omitted, using an appropriate statistical method (e.g. Median Absolute Deviation, MAD).
The RE for QC samples should be within ±30% of the nominal value, except for LLOQ‐QC where RE should be within ±50% of the nominal value.
Negative extraction controls, no‐template controls, and sentinel controls should not have VG copies greater than LOD.
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copies/reactions in the same spike volume added to a non‐interfering matrix or plain buffer. If the Cq or Ct value of the spiked test sample replicate is significantly higher than that of the SVC, sample‐mediated inhibition has occurred and ade­quate remediation procedures may be needed, as detailed in Table7.4.
Table7.4  Exemplary test sample acceptance criteria forPCR.
Acceptance criteria qPCR sample 
Parameter
Sample‐ mediated inhibition
Precision Samples should be tested in
Sample result determination
analysis
Samples should be non‐inhibitory. Samples are considered inhibitory if the spiked replicate exhibits a Cq or Ct value that is greater than
1.73
+ the mean Cq or Ct value of the spike volume control (SVC). Inhibitory samples shall be retested. If a sample remains inhibitory upon retest, it should be reextracted from biological specimens. If re‐extraction is not feasible, the sample should be reported as not determinable (ND).
triplicate or duplicate. The CV of replicate Cq or Ct values for a sample above the LLOQ should be ≤3%. OR: the CV of interpolated VG copies for a sample above the LLOQ should be ≤30%. Outlier exclusion: 1 out of 3 replicates may be omitted, if the Cq or Ct differences between the omitted replicate and each ofthe other two replicates are greater than 1.
Samples are considered positive if the level of fluorescence after 40 cycles rises above the run‐specific threshold and is indicative of true amplification. Otherwise, samples are considered negative. OR: Samples are considered positive if interpolated mean VG copies fall at or above the LOD. Otherwise, samples are considered negative.
Acceptance criteria dPCR sample 
analysis
Samples should be non‐inhibitory. Samples are considered inhibitory if the spiked replicate exhibits measured VG copies that are more than 30% lower than those in the SVC. Inhibitory samples shall be retested. If a sample remains inhibitory upon retest, it should be reextracted from biological specimens. If re‐extraction is not feasible, the sample should be reported as ND.
Samples should be tested in triplicate or duplicate. Samples with mean VG copies above 2 times the LLOQ must have CV ≤30%. Samples with mean VG copies between 2 times the LLOQ and the LLOQ must have CV ≤50%. Outlier exclusion: 1 out of 3 replicates may be omitted, using appropriate statistical methods (e.g., Median Absolute Deviation, MAD).
Samples are considered positive if mean VG copies fall at or above the LOD. Otherwise, samples are considered negative.
         187
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Parameter
Reporting ranges
Acceptance criteria qPCR sample 
analysis
Positive samples should be quantified and reported with a numerical result if they fall within the limits of quantification (LLOQ, ULOQ) established in assay validation. Samples above the ULOQ should be diluted adequately and retested. Positive samples below the LLOQ should be reported as falling below the lower limit of quantification (BLQ). Negative samples should be reported as 0 VG copies or below the limit of detection (BLD).
Acceptance criteria dPCR sample 
analysis
Positive samples should be quantified and reported with a numerical result if they fall within the limits of quantification (LLOQ, ULOQ) established in assay validation. Samples above the ULOQ should be diluted adequately and retested. Positive samples below the LLOQ should be reported as falling BLQ. Negative samples should be reported as 0 VG copies or BLD.
Routine clinical vector shedding assays should be monitored for consistent per­formance. For qPCR methods, the Cq or Ct values of the standard curve could be trended and should typically fall within two standard deviations from the mean. VG copies for QCs can also be trended, which may be the most appropriate way to monitor ddPCR performance over time. The proportion of droplets that pass internal instrument quality checks may also be monitored in ddPCR since a sig­nificant and prolonged change in accepted droplets used for quantification could impact assay performance.
7.11   Immunocapture qPCR: AnUltra-Sensitive  Method toDetect Intact AAV Capsids
The main objectives of clinical vector shedding studies are to evaluate the poten­tial risk of horizontal transmission and release into the environment. AAV‐based GTx are generally designed to have minimal risk, given that they are replication‐ incompetent, non‐pathogenic, encode nontoxic gene products, have weak or moderate promoters/enhancers, do not carry antibiotic resistance genes, and rarely integrate into the host genome. Nonetheless, there is a desire to limit any unintended exposure or release. Monitoring vector shedding can inform the dura­tion of potential pre‐cautionary measures and is therefore mandated by regula­tory agencies during clinical studies.
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If standard PCR‐based methods are used to monitor shedding, encapsidated and non‐encapsidated vector DNA cannot be distinguished. This distinction, however, could be informative, since the potential risk associated with each of these two forms of vector DNA is not equal. On the one hand, encapsidated vector DNA is contained in structurally intact capsid particles that may have the capabil­ity to enter cells, i.e. this form of vector DNA is potentially transduction compe­tent. On the other hand, non‐encapsidated vector DNA could stem from low amounts of degraded capsid particles or emerge during uncoating within trans­duced cells and be released following cell death. No meaningful transduction competence would therefore be expected for non‐encapsidated vector DNA, which decreases any potential risk.
To measure intact GTx vector capsids, cell‐based transduction assays or ELISA‐ based immunoassays may be used[3, 28]. In clinical practice, however, the limited sensitivity of these methods poses a significant challenge to detecting low residual levels of GTx capsids in shedding matrices or blood[29, 30]. Recently, a more sensi­tive ELISA‐based method named VIRELISA was described, which employs a recombinant high‐affinity AAV receptor (AAVR) to capture and detect AAV2 cap­sids [31]. With an estimated sensitivity of detecting as low as 5E+05 of AAV2 capsids in buffered aqueous solution, this method may warrant further investigation, even though the sensitivity for matrixed clinical samples and other AAV serotypes remains unclear.
To overcome the hurdles imposed by limited sensitivity of existing methods, an ultra‐sensitive assay for detecting structurally intact, and thus potentially transduction‐ competent capsids has recently been developed for an AAV5‐based GTx [3]. This two‐step methodology, named immunocapture‐qPCR (iqPCR), features a monoclo­nal antibody (ADK5a/b) that specifically recognizes a conformational epitope on AAV5 capsids and facilitates magnetic bead‐mediated pull‐down, followed by capsid denaturation and PCR amplification of released vector DNA. The major advantage of iqPCR is the quantum leap in analytical sensitivity for detecting AAV capsids across various clinical matrices. IqPCR exceeds the sensitivity of both cell‐based methods and advanced immunoassay platforms (including single‐molecule count­ing, or SMC‐Erenna), and approximates detection limits of standard PCR‐based methods. Compared to VIRELISA, iqPCR is at least 40 times more sensitive.
Another advantage of iqPCR over ELISA‐based methods is its dually assured specificity for intact GTx capsids, (1) by using a conformation‐specific antibody for a particular AAV serotype and (2) by ensuring that captured capsids contain specific vector DNA. ELISA‐based methods, such as VIRELISA, detect AAV cap­sids regardless of whether they are full (“heavy”) or empty (“light”), or regardless of whether they contain vector DNA or a wild‐type AAV genomes. Hence, one may expect potential interference from natural AAV infections for ELISA‐based capsid detection methods if used in clinical studies.
vg/mL
References 189
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A notable limitation of iqPCR (as well as ELISA‐based capsid detection meth­ods) is interference from AAV antibodies. These antibodies may compete for epitopes with the monoclonal antibody reagent used to capture capsids, and conse­quently iqPCR has a low tolerance to AAV antibodies present in plasma or semen[3]. This interference is of lesser concern within the first few days after GTx administration, since patients have usually been pre‐screened to have low or no pre‐existing humoral immunity to AAV. While treatment‐emergent AAV antibod­ies can impact capsid quantification in iqPCR, they are generally also highly neu­tralizing[32–34]. Hence, AAV capsid/antibody complexes that remain undetectable by iqPCR would also be less likely to mediate horizontal transmission or environ­mental impact, as compared to “free” transduction‐competent AAV capsids.
IqPCR has been successfully implemented in clinical GTx studies and used to describe clearance kinetics of transduction‐competent “free” GTx vector in human biological fluids[3]. This helped inform the duration of precautionary measures deemed necessary to limit unintended exposure or release. In summary, PCR‐based methods to detect vector DNA in tissues and biological fluids are essential components of bioanalytical portfolios that support the development of novel AAV GTx.
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