CytomX Therapeutics Announces Second Quarter 2020 Financial Results and Provides Business Update Company to Host a Conference Call Today

CytomX Therapeutics Announces Second Quarter 2020 Financial Results

and Provides Business Update

Company to Host a Conference Call Today, August 6, 2020, at 5:30 p.m. ET / 2:30 p.m. PT

SOUTH SAN FRANCISCO, CA, August 6, 2020- CytomX Therapeutics, Inc. (Nasdaq: CTMX), a clinical-stage oncology-focused biopharmaceutical company pioneering a novel class of investigational antibody therapeutics based on its Probody technology platform, today reported second quarter 2020 financial results and provided a business update.

"CytomX made broad progress across our clinical and preclinical programs during the second quarter as we further advanced our technology platform, partnerships, and lead drug candidates to several important inflection points," said Sean McCarthy, D.Phil., president, chief executive officer and chairman of CytomX Therapeutics. "We continue to show leadership in defining new therapeutic antibody modalities with demonstrated potential to address significant unmet medical needs in the treatment of cancer, a mission that remains as important as ever despite the unprecedented challenges being posed by the COVID-19 pandemic."

Second Quarter Business Highlights and Recent Developments

Clinical Pipeline Progress: ASCO20 Data Presentations and Program Next Steps

In May, CytomX presented broad clinical pipeline progress at the American Society of Clinical Oncology ASCO20 Virtual Scientific Program from four Probody programs advancing in, or towards, Phase 2 studies:

CX-2029 Phase 1 Data: Preliminary Validation of CD71 as a Novel Oncology Target and Advancement to Phase 2 Expansion Cohorts

CX-2009: Phase 2 Strategies Focused in HER2 Negative Breast Cancer Subtypes

BMS-986249: Anti-CTLA-4 Probody Immunotherapeutic

During the second quarter, Bristol Myers Squibb also presented a comprehensive preclinical dataset from the anti-CTLA-4 Probody immunotherapeutics, BMS-986249 and BMS-986288, a non-fucosylated Probody of ipilimumab, at the American Association of Cancer Research's

(AACR) 2020 Virtual Annual Meeting II. These data support the strategy of expanding the therapeutic index for CTLA-4 therapy using CytomX's Probody technology and provides rationale for the ongoing clinical studies of these agents.

CX-072: Anti-PD-L1 Probody Immunotherapeutic

Preclinical Pipeline Progress

CX-904 EGFR-CD3 Probody Bispecific

CX-2043 EpCAM Probody Drug Conjugate

Astellas Collaboration

COVID-19 Pandemic and Business Continuity

CytomX is committed to ensuring the health, safety and well-being of its clinical study participants, study site staff, and our employees. CytomX continues to closely monitor the COVID-19 pandemic situation and is following local, state, and federal guidelines, including emerging Health Authority guidance and IRB/Ethics Committee recommendations with respect to the conduct of our worldwide clinical trials. In accordance with state and local guidelines, CytomX is following a work-from-home protocol for many of its employees with only select

staff, including those in research functions that require laboratory access, having access to CytomX's corporate offices where multiple layers of safety measures have been put in place.

Second Quarter 2020 Financial Results

Cash, cash equivalents and short-term investments totaled $346.4 million as of June 30, 2020, compared to $296.1 million as of December 31, 2019.

Revenue was $16.6 million for the three months ended June 30, 2020, compared to $9.0 million for the three months ended June 30, 2019. The net increase in revenue of $7.6 million was primarily due to an increase of $3.4 million from the Amgen EGFR project as a result of a higher percentage of completion progress in the second quarter of 2020, and an increase of $4.2 million related to the recognition of revenue from the $80 million upfront payment under the Collaboration and License Agreement with Astellas entered into in March 2020.

Research and development expenses decreased by $6.8 million during the three months ended June 30, 2020 compared to that in the corresponding period in 2019. The decrease was largely attributed to $3.4 million for the University of California, Santa Barbara (UCSB) fees paid in 2019 relating to the amendment to the license agreement, and a $0.8 sublicense fee also paid to UCSB in 2019 relating to a $10.0 million milestone earned by the Company for AbbVie's selection of a second target under the Amended and Restated Discovery Collaboration and License Agreement with AbbVie in 2019 and a decrease of $2.3 million in clinical related expenses due to the decrease in clinical trial activities.

General and administrative expenses decreased by $0.7 million during the three months ended June 30, 2020 compared to that in the corresponding period in 2019 primarily due to a decrease in stock base compensation expense.

Teleconference Scheduled Today at 5:30 p.m. ET

Conference Call/Webcast Information

CytomX management will host a conference call today at 5:30 p.m. ET. Interested parties may access the live audio webcast of the teleconference through the "Investor & News" section of CytomX's website at http://ir.cytomx.com or by dialing 1-877-809-6037 (U.S. and Canada) or 1-615-247-0221 (International) and using the passcode 5399347. An archive of the webcast will be available on the CytomX website from August 6, 2020, until August 20, 2020.

About CytomX Therapeutics

CytomX is a clinical-stage, oncology-focused biopharmaceutical company with a vision of transforming lives with safer, more effective therapies. We are developing a novel class of investigational antibody therapeutics, based on our Probody technology platform, for the treatment of cancer. CytomX has strategic drug discovery and development collaborations with

AbbVie, Amgen, Astellas and Bristol Myers Squibb.

CytomX Therapeutics Forward-Looking Statements

This press release includes forward-looking statements. Such forward-looking statements involve known and unknown risks, uncertainties and other important factors that are difficult to predict, may be beyond our control, and may cause the actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied in such statements. Accordingly, you should not rely on any of these forward-looking statements, including those relating to the potential benefits, safety and efficacy or progress of CytomX's or any of its collaborative partners' product candidates, the potential benefits or applications of CytomX's Probody platform technology, and CytomX's ability to develop and advance product candidates into and successfully complete clinical trials, including the ongoing and planned clinical trials of CX-2009 and CX-2029. Risks and uncertainties that contribute to the uncertain nature of the forward-looking statements include: the unproven nature of CytomX's novel Probody Platform technology; CytomX's clinical trial product candidates are in the initial stages of clinical development and its other product candidates are currently in preclinical development, and the process by which preclinical and clinical development could potentially lead to an approved product is long and subject to significant risks and uncertainties, including the risk that the COVID-19 worldwide pandemic may continue to negatively impact the business, research and clinical operations of CytomX or its partners, including the development of preclinical drug candidates due to delays in and

disruption of research activities and the development of clinical drug candidates due to delays in or disruption of clinical trials, including impacts on the enrollment of patients in clinical trials or other clinical trial disruptions; the possibility that the results of early clinical trials may not be predictive of future results; the possibility that CytomX's clinical trials will not be successful; the possibility that current pre-clinical research may not result in additional product candidates; CytomX's dependence on the success of CX-2009, CX-2029, BMS-986249, BMS-986288, and CX-072; CytomX's reliance on third parties for the manufacture of the company's product candidates; and possible regulatory developments in the United States and foreign countries. Additional applicable risks and uncertainties include those relating to our preclinical research and development, clinical development, and other risks identified under the heading "Risk Factors" included in CytomX's Quarterly Report on Form 10-Q filed with the SEC on August 6, 2020. The forward-looking statements contained in this press release are based on information currently available to CytomX and speak only as of the date on which they are made. CytomX does not undertake and specifically disclaims any obligation to update any forward-looking statements, whether as a result of any new information, future events, changed circumstances or otherwise.

Probody is a U.S. registered trademark of CytomX Therapeutics, Inc.

Yervoy and Opdivo are registered trademarks of Bristol Myers Squibb.

CYTOMX THERAPEUTICS, INC.

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS

(in thousands, except share and per share data)

CYTOMX THERAPEUTICS, INC.

CONDENSED BALANCE SHEETS

(in thousands, except share and per share data)