Castle Biosciences Presents Data Demonstrating DecisionDx-Melanoma as Significant, Independent Predictor of Recurrence in Stage I Cutaneous Melanoma Population

Castle Biosciences, Inc. (Nasdaq: CSTL), a skin cancer diagnostics company providing personalized genomic information to improve cancer treatment decisions, will present data at the American Academy of Dermatology's Virtual Meeting Experience, being held April 23 ? 25, 2021. The Company has two poster presentations highlighting its DecisionDx®-Melanoma and DecisionDx®-SCC genomic tests.

Data presentation information is as follows:

DecisionDx®-Melanoma:

The poster is entitled, "Risk stratification of patients with stage I cutaneous melanoma (CM) using 31-gene expression profiling (GEP)."

DecisionDx-Melanoma is Castle's 31-gene expression profile test that uses an individual patient's tumor biology to predict risk of cutaneous melanoma metastasis or recurrence, as well as sentinel lymph node (SLN) positivity, independent of traditional staging factors.

Study methods and findings:

• Data were analyzed from 852 patients diagnosed with Stage I cutaneous melanoma according to the American Joint Committee on Cancer staging guidelines and a median follow-up of 3.8 years, who also had DecisionDx-Melanoma test results. Kaplan-Meier and Cox regression analyses were used to assess recurrence-free survival (RFS). Prognostic accuracy was assessed by comparing outcomes for Class 1A to outcomes for Class 2B.
• The study demonstrated that along with staging factors (Breslow thickness and ulceration), a DecisionDx-Melanoma Class 2B (highest risk) result was a significant, independent predictor of recurrence in the stage I population.
  • A Class 2B DecisionDx-Melanoma result among these patients was associated with a recurrence rate of 20.4%, compared to the overall stage I population's recurrence risk of 4.6%.
  • Among all stage I melanoma patients, those with a Class 2B DecisionDx-Melanoma result had seven times the odds of experiencing a recurrence than patients with a Class 1A result.
  • In the subgroup of patients with a confirmed negative sentinel lymph node biopsy, those with a Class 2B DecisionDx-Melanoma result had five times the odds of experiencing a recurrence than patients with a Class 1A result.
• Although the stage I population has a low aggregate risk of recurrence, the study demonstrated that DecisionDx-Melanoma identified patients within that population who may experience recurrence and benefit from increased management intensity.

DecisionDx®-SCC:

The poster is entitled, "Prospective adjuvant therapy trial design using a prognostic 40-gene expression profile (40-GEP) test for high-risk cutaneous squamous cell carcinoma (cSCC) and BWH staging-based risk assessment."

DecisionDx-SCC is Castle's prognostic 40-gene expression profile test for patients diagnosed with high-risk cutaneous squamous cell carcinoma (SCC) designed to use a patient's tumor biology to predict individual risk of metastasis for patients with SCC and one or more risk factors.

Study methods and findings:

• This analysis evaluated the impact of DecisionDx-SCC's risk stratification as enrollment criteria into trial design involving adjuvant therapy for SCC.
• Using data from 420 validation cases, Brigham and Women's Hospital (BWH) T2a-T3 stage SCC patients with or without DecisionDx-SCC results of Class 2A/B (high risk) or 2B (highest risk) were used for two-arm sample size calculations.
• Metastasis rates for cases with BWH T2a-T3 tumors were 20% without DecisionDx-SCC results and 27% or 57% when selecting for cases with Class 2A/B or 2B results, respectively.
• In the absence of DecisionDx-SCC results, 1,234 T2a-T3 patients would be required for randomization in a trial to provide 80% power to detect a hazard ratio of 0.7 with at least three years of follow-up (alpha=0.05), correlating with studies using the addition of radiation to surgery. However, sample size could be reduced to 915 (26% reduction) or 432 (65% reduction) patients by focusing enrollment on T2a-T3 patients with a DecisionDx-SCC Class 2A/B or 2B result, respectively.
• Use of DecisionDx-SCC could be applied to improve trial inclusion criteria as it has demonstrated additive value for patient stratification in other risk assessment methods such as AJCC staging, NCCN risk groups, and individual risk factors.
• Overall, the study demonstrated that incorporation of DecisionDx-SCC testing into trial design to identify patients who are at the highest risk for metastasis could facilitate selection of those who are most appropriate for adjuvant therapy (e.g., Class 2B patients), expedite time to trial completion and optimize healthcare costs.

About DecisionDx-Melanoma

DecisionDx®-Melanoma is a gene expression profile test that uses an individual patient's tumor biology to predict individual risk of cutaneous melanoma metastasis or recurrence, as well as sentinel lymph node positivity, independent of traditional staging factors, and has been studied in more than 5,700 patient samples. Using tissue from the primary melanoma, the test measures the expression of 31 genes. The test has been validated in four archival risk of recurrence studies of 901 patients and six prospective risk of recurrence studies including more than 1,600 patients. To predict likelihood of sentinel lymph node positivity, the Company utilizes its proprietary algorithm, i31-GEP, to produce an integrated test result. i31-GEP is an artificial intelligence-based neural network algorithm (independently validated in a cohort of 1,674 prospective, consecutively tested patients with T1-T4 cutaneous melanoma) that integrates the DecisionDx-Melanoma test result with the patient's traditional clinicopathologic features. Impact on patient management plans for one of every two patients tested has been demonstrated in four multicenter and single-center studies including more than 560 patients. The consistent performance and accuracy demonstrated in these studies provides confidence in disease management plans that incorporate DecisionDx-Melanoma test results. Through December 31, 2020, DecisionDx-Melanoma has been ordered more than 68,920 times for use in patients with cutaneous melanoma.

More information about the test and disease can be found at www.CastleTestInfo.com.

About DecisionDx-SCC

DecisionDx-SCC is a 40-gene expression profile test that uses an individual patient's tumor biology to predict individual risk of cutaneous squamous cell carcinoma metastasis for patients with one or more risk factors. The test result, in which patients are stratified into a Class 1, 2A or 2B risk category, predicts individual metastatic risk to inform risk-appropriate management.

Peer-reviewed publications have demonstrated that DecisionDx-SCC is an independent predictor of metastatic risk and that integrating DecisionDx-SCC with current prognostic methods can add positive predictive value to clinician decisions regarding staging and management.

More information about the test and disease can be found at www.CastleTestInfo.com.

About Castle Biosciences

Castle Biosciences (Nasdaq: CSTL) is a commercial-stage dermatologic cancer company focused on providing physicians and their patients with personalized, clinically actionable genomic information to make more accurate treatment decisions. The Company currently offers tests for patients with cutaneous melanoma (DecisionDx®-Melanoma, DecisionDx®-CMSeq), cutaneous squamous cell carcinoma (DecisionDx®-SCC), suspicious pigmented lesions (DecisionDx® DiffDxtm-Melanoma) and uveal melanoma (DecisionDx®-UM, DecisionDx®-PRAME and DecisionDx®-UMSeq). For more information about Castle's gene expression profile tests, visit www.CastleTestInfo.com. Castle also has active research and development programs for tests in other dermatologic diseases with high clinical need. Castle Biosciences is based in Friendswood, Texas (Houston), and has laboratory operations in Phoenix, Arizona. For more information, visit www.CastleBiosciences.com.

DecisionDx-Melanoma, DecisionDx-CMSeq, DecisionDx-SCC, DecisionDx DiffDx-Melanoma, DecisionDx-UM, DecisionDx-PRAME and DecisionDx-UMSeq are trademarks of Castle Biosciences, Inc.

Forward-Looking Statements

The information in this press release contains forward-looking statements and information within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended, which are subject to the "safe harbor" created by those sections. These forward-looking statements include, but are not limited to, statements concerning DecisionDx-Melanoma's ability to predict recurrence in the stage I population and identify patients within that population who may benefit from increased management intensity and statements concerning the ability of DecisionDx-SCC test results to provide additive value for patient stratification in other risk assessment methods such as AJCC staging, NCCN risk groups, and individual risk factors, identify patients who are at the highest risk for metastasis, facilitate selection of those who are most appropriate for adjuvant therapy (e.g., Class 2B patients), expedite time to trial completion and optimize healthcare costs. The words "anticipates," "believes," "estimates," "expects," "intends," "may," "plans," "projects," "will," "would" and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. We may not actually achieve the plans, intentions, or expectations disclosed in our forward-looking statements and you should not place undue reliance on our forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements that we make. These forward-looking statements involve risks and uncertainties that could cause our actual results to differ materially from those in the forward-looking statements, including, without limitation, the effects of the COVID-19 pandemic on our business and our efforts to address its impact on our business, subsequent study results and findings that contradict earlier study results and findings, our products' ability to provide the aforementioned benefits to patients and the risks set forth in our Annual Report on Form 10-K for the year ended December 31, 2020, and in our other filings with the SEC. The forward-looking statements are applicable only as of the date on which they are made, and we do not assume any obligation to update any forward-looking statements, except as may be required by law.