Sutro Biopharma Reports First Quarter 2020 Financial Results and Recent Business Highlights and Developments STRO-002 Encouraging Interim Phase 1 Clinical Data from an Ongoing Dose Escalation Study in Ovarian Cancer Pres

Sutro Biopharma Reports First Quarter 2020 Financial Results

and Recent Business Highlights and Developments

STRO-002 Encouraging Interim Phase 1 Clinical Data from an Ongoing Dose Escalation Study in Ovarian Cancer Presented at the AACR Virtual Meeting

STRO-001 Phase 1 Clinical Trial and Dose Escalation Ongoing in Multiple Myeloma and Lymphoma

Merck Extends Research Term of Collaboration's First Cytokine-Derivative Program

Sutro Unveils Innovative Cancer Therapy Approach Using Precise Tumor Targeted Immunostimulant Antibody-Drug Conjugate at World ADC London

SOUTH SAN FRANCISCO, Calif., May 11, 2020 - Sutro Biopharma, Inc. (NASDAQ: STRO), a clinical-stage drug discovery, development and manufacturing company focused on the application of precise protein engineering and rational design to create next-generation oncology therapeutics, today reported its financial results for the quarter ended March 31, 2020 and its recent business highlights and developments.

"We are pleased with the encouraging dose escalation safety and anti-tumor activity data from our Phase 1 clinical trial for STRO-002, including data presented during the AACR Virtual Meeting on April 27, 2020," said Bill Newell, Sutro's Chief Executive Officer. "These STRO-002 data demonstrate preliminary evidence of anti-tumor activity, particularly in a heavily pre-treated patient population along with an emerging safety profile that indicates that the product candidate is generally well tolerated. Our two proprietary antibody-drug conjugate (ADC) product candidates, STRO-001 and STRO-002, are progressing in Phase 1 clinical trials. With the ongoing COVID-19 pandemic, Sutro is committed to the health and safety of patients receiving our therapies and our employees and expects our trials and drug supply to remain on track while reducing the risks to our employees as much as possible. Additionally, each of our three current collaborations has yielded a novel oncology product candidate in clinical development or in the late stages of preclinical development, all of which were discovered, developed, and are manufactured using our proprietary and integrated cell-free protein synthesis platform XpressCF and site-specific conjugation platform XpressCF+ ."

Recent Business Highlights and Developments

STRO-002 Clinical Program

STRO-001 Clinical Program

Cytokine-Derivative Programs (Collaboration with Merck & Co.)

BCMA ADC Clinical Program (Collaboration with Bristol Myers Squibb; formerly Celgene)

Bispecific ADC Clinical Development Candidate (Collaboration with EMD Serono)

24-Valent Pneumococcal Conjugate Vaccine (SutroVax Relationship)

Sutro Unveils Innovative Cancer Therapy Approach Using Precise Tumor Targeted Immunostimulant ADCs at World ADC London

First Quarter 2020 Financial Highlights

Cash, Cash Equivalents and Marketable Securities

As of March 31, 2020, Sutro had cash, cash equivalents and marketable securities of $129.6 million, as compared to $133.5 million as of December 31, 2019, which represents net cash usage of $3.9 million during the first quarter of 2020.

On February 28, 2020, Sutro entered into a loan and security agreement, under which Sutro borrowed $25.0 million, with approximately $9.6 million of such amount used to repay the outstanding principal, interest and final payment fees under a prior loan with the same lenders.

Revenue was $7.2 million for the quarter ended March 31, 2020, compared to $8.6 million in the corresponding 2019 quarter, principally related to the Merck, BMS, and EMD Serono collaborations. Future collaboration revenue from Merck, BMS, and EMD Serono, and from any future collaboration partners, will fluctuate as a result of the amount and timing of revenue recognition of upfront, milestones and other collaboration agreement payments.

Total operating expenses for the quarter ended March 31, 2020 were $26.3 million, compared to $22.9 million in the corresponding 2019 quarter, including non-cash stock-based compensation of $2.7 million and $2.3 million, and depreciation and amortization expense of $1.1 million and $1.1 million, in the 2020 and 2019 quarters, respectively. Total operating expenses for the first quarter 2020 were comprised of research and development expenses of $17.6 million and general and administrative expenses of $8.7 million, with both expense types expected to increase in 2020 as Sutro's internal product candidates advance in clinical development and additional general and administrative expenses are incurred as a public company.

About Sutro Biopharma

Sutro Biopharma, Inc., located in South San Francisco, is a clinical-stage drug discovery, development and manufacturing company. Using precise protein engineering and rational design, Sutro is advancing next-generation oncology therapeutics.

Sutro's proprietary and integrated cell-free protein synthesis platform XpressCF and site-specific conjugation platform, XpressCF+ , led to the discovery of STRO-001 and STRO-002, Sutro's first two internally-developed ADCs. STRO-001 is a CD74-targeting ADC currently being investigated in a Phase 1 clinical trial of patients with advanced B-cell malignancies, including multiple myeloma and non-Hodgkin lymphoma. STRO-001 was granted Orphan Drug Designation by the FDA for multiple myeloma in October 2018. STRO-002 is a folate receptor alpha (FolR )-targeting ADC, currently being investigated in a Phase 1 clinical trial of patients with ovarian and endometrial cancers. This is the second product candidate to be evaluated in clinical trials resulting from Sutro's XpressCF and XpressCF+ technology platforms. A third program, CC-99712 (BCMA-targeting ADC), which is part of Sutro's collaboration with Bristol Myers Squibb (formerly Celgene Corporation), is enrolling patients for its Phase 1 clinical trial of patients with multiple myeloma. Sutro's proprietary technology was responsible for the discovery and manufacturing of CC-99712, for which Bristol Myers Squibb has worldwide development and commercialization rights. Sutro is entitled to development and regulatory milestone payments and tiered royalties from Bristol Myers Squibb for this BCMA ADC. Sutro is dedicated to transforming the lives of cancer patients by creating medicines with improved therapeutic profiles for areas of unmet need.

To date, Sutro has designed cytokine-based immuno-oncology therapies, ADCs, vaccines and bispecific antibodies primarily directed at clinically-validated targets for which the current standard of care is suboptimal.

Sutro's platform allows it to accelerate discovery and development of potential first-in-class and best-in-class molecules through rapid and systematic evaluation of protein structure-activity relationships to create optimized homogeneous product candidates.

In addition to developing its own oncology pipeline, Sutro is collaborating with select pharmaceutical and biotech companies to discover and develop novel, next-generation

therapeutics. As the pace of clinical development accelerates, Sutro and its partners are developing therapeutics designed to more efficiently kill tumors without harming healthy cells.

Additional multimedia content from Sutro regarding STRO-001 and STRO-002 can be found here and here.

Follow Sutro on Twitter, @Sutrobio, and at www.sutrobio.com to learn more about our passion for changing the future of oncology.

Forward-Looking Statements

This press release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995, including, but not limited to, anticipated preclinical and clinical development activities, timing of announcements of clinical results, potential benefits of the company's product candidates and platform and potential market opportunities for the company's product candidates. All statements other than statements of historical fact are statements that could be deemed forward-looking statements. Although the company believes that the expectations reflected in such forward-looking statements are reasonable, the company cannot guarantee future events, results, actions, levels of activity, performance or achievements, and the timing and results of biotechnology development and potential regulatory approval is inherently uncertain. Forward-looking statements are subject to risks and uncertainties that may cause the company's actual activities or results to differ significantly from those expressed in any forward-looking statement, including risks and uncertainties related to the company's ability to advance its product candidates, the receipt and timing of potential regulatory designations, approvals and commercialization of product candidates, the impact of the COVID-19 pandemic on the Company's business, clinical trial sites, supply chain and manufacturing facilities, the Company's ability to maintain and recognize the benefits of certain designations received by product candidates, the timing and results of preclinical and clinical trials, the company's ability to fund development activities and achieve development goals, the company's ability to protect intellectual property, and the Company's commercial collaborations with third parties and other risks and uncertainties described under the heading "Risk Factors" in documents the company files from time to time with the Securities and Exchange Commission. These forward-looking statements speak only as of the date of this press release, and the company undertakes no obligation to revise or update any forward-looking statements to reflect events or circumstances after the date hereof.

Sutro Biopharma, Inc.

Selected Statements of Operations Financial Data

(In thousands, except share and per share amounts)

Sutro Biopharma, Inc.

Selected Balance Sheet Financial Data