UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): February 7, 2020

CATALYST BIOSCIENCES, INC.

(Exact name of registrant as specified in its charter)

611 Gateway Blvd, Suite 710, South San Francisco, CA 94080

(Address of principal executive offices)

(650) 871-0761

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company  ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.  ☐

Item 7.01 Regulation FD Disclosure.

On February 7, 2020, Catalyst Biosciences, Inc. (the “Company”) posted an update to its corporate presentation (the “Presentation”) on its website, ir.catalystbiosciences.com/presentations-events. A copy of the Presentation is attached hereto as Exhibit 99.1.

On February 7, 2020, the Company also announced positive data from its Phase 2b trial of subcutaneous dalcinonacog alfa (DalcA) and marzeptacog alfa (MarzAA). The data were presented at the European Association for Haemophilia and Allied Disorders (EAHAD) in The Hague, Netherlands and are summarized in the Company’s press release issued on February 7, 2020. The press release also announced the details for the conference call that the Company’s management team is scheduled to host for investors on February 7, 2020 to discuss the DalcA and MarzAA data presented at EAHAD. A copy of the press release is attached hereto as Exhibit 99.2.

The information in this Item 7.01 of this Current Report on Form 8-K (including Exhibits 99.1 and 99.2) is being furnished and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section. The information in this Current Report shall not be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such a filing.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Exhibit 99.1

CATALYST BIOSCIENCES Corporate Overview 7 February 2020

Forward looking statements Nasdaq: CBIO This presentation includes forward-looking statements Various important factors could cause actual results or that involve substantial risks and uncertainties All events to differ materially, including, but not limited to, the statements included in this presentation, other than risk that additional human trials will not replicate the statement of historical facts, are forward-looking results from earlier trials or animal studies, that potential statements Examples of such statements include, but adverse effects may arise from the testing or use of are not limited to, potential markets for MarzAA, DalcA MarzAA or DalcA, including the generation of antibodies, and CB 2782-PEG, potential benefits of subcutaneous which has been observed in patients treated with DalcA, dosing, potential use of MarzAA as a subcutaneous that clinical trials will take longer than anticipated to be therapy for patients with hemophilia A or B with inhibitors completed, that costs required to develop or manufacture and other bleeding disorders, potential use of DalcA as a the Company’s products will be higher than anticipated, subcutaneous therapy for patients with hemophilia B, that Biogen will discontinue development of CB 2782-potential benefits of CB 2679d-GT as gene therapy, PEG, competition and other factors that affect our ability clinical trial results, plans for a registrational trial for to establish collaborations on commercially reasonable MarzAA in second half of 2020, plans for final Phase 2b terms and other risks described in the “Risk Factors” clinical trial data for DalcA in the second quarter of 2020, section of the Company’s quarterly report on Form 10-Q plans for non-human primate data for CB 2679d-GT in the filed with the Securities and Exchange second quarter of 2020, and potential milestone and Commission on November 7, 2019, and in other filings royalty payments from Biogen Actual results or events with the Securities and Exchange Commission The could differ materially from the plans, expectations and Company does not assume any obligation to update any projections disclosed in these forward-looking statements forward-looking statements, except as required by law catalystbiosciences com 2

Nasdaq: CBIO Essential Medicines – Superior Outcomes Late-Stage Asset Hemophilia Complement SQ Marzeptacog alfa SQ MarzAA IVT Anti-C3 (activated) CB 2782-PEG MarzAA (FVIIa) SQ Dalcinonacog alfa – DalcA (FIX) Phase 3 Ready Factor IX Gene Therapy SQ Systemic Complement Factor Xa Inhibitors Protease Engineering Platform catalystbiosciences com 3

Pipeline Nasdaq: CBIO Hemostasis R PC P1/2 P2 P3 SQ Marzeptacog alfa (activated) “MarzAA” Hemophilia & bleeding disorders (rFVIIa) SQ Dalcinonacog alfa “DalcA” Hemophilia B (rFIX) FIX-Gene Therapy Hemophilia B (CB 2679d-GT) Complement IVT CB 2782-PEG anti-C3 protease for Dry AMD SQ Systemic complement inhibitors catalystbiosciences com 4

Investment highlights Nasdaq: CBIO Novel subcutaneous factors with orphan drug designation, Multi-billion-dollar market FVIIa opportunities MarzAA & DalcA – SQ P2b FIX clinical efficacy demonstrated Anti-C3 collaboration with Biogen Experienced team SQ systemic complement inhibitors research program ~134 worldwide patents – Well funded CBIO retains full ownership $85 M cash (Q3 2019) of all compounds catalystbiosciences com 5

Addressing unmet needs in orphan bleeding disorders Nasdaq: CBIO SQ treatment of bleeds and prophylaxis – $3 7B market Hemophilia A MarzAA & DalcA w/ Inhibitors Hemophilia B Hemophilia B Hemophilia A w/ Inhibitors Factor VII Acquired Deficiency Hemophilia Glanzmann Thrombasthenia catalystbiosciences com 6

The Catalyst Biosciences subcutaneous solution Nasdaq: CBIO Our highly potent candidates + Quick & simple self-administered SQ injection + SQ dosing is the future in hemophilia and other rare hematology indications + Ideal for pediatric patients + Significantly increases half-life + Much higher & more stable factor levels for prophylaxis + Enable SQ treatment of bleeding catalystbiosciences com 7

The new standard in hemophilia prophylaxis Nasdaq: CBIO Patients in high mild range are protected from spontaneous bleeds Normal clotting levels IV treatment SQ treatment 50% Mild Hemophilia Protection from spontaneous hemarthrosis when activity >12% 5% Moderate Hemophilia 15-20 bleeds/year 1% Severe Hemophilia ~30 bleeds/year + Our concept of prophylactic treatment is to keep severe & moderate hemophilia patients in the high mild range + Subcutaneous factor treatments build up over time, offering long-term stability in clotting levels catalystbiosciences com 8

MarzAA is only bypass agent for both SQ prophylaxis and Nasdaq: CBIO SQ treatment of bleeds Attractive commercial profile targeting an existing $2 2B bypass agent market IV NovoSeven ($1 2B 2018 sales) validates SQ MarzAA has a superior profile to IV rFVIIa in multiple rare bleeding disorders NovoSeven – over 100 clinicians surveyed: + Hemophilia A or B with inhibitors + All physicians surveyed indicated a + Severe Factor VII Deficiency preference for SQ MarzAA over IV N7 in + Glanzmann Thrombasthenia one or more settings + Acquired Hemophilia A + SQ MarzAA can create & expand multiple prophylaxis markets catalystbiosciences com 9

Marzeptacog alfa (activated): MarzAA rFVIIa Nasdaq: CBIO SQ prophylaxis and SQ treatment of a bleed are clear unmet needs in hemophilia and other bleeding disorders + Four engineered amino acid substitutions P129A T128N within the FVIIa protein + 9-fold more potent catalytic activity than NovoSeven RT M298Q + Allows subcutaneous dosing + Half-life prolonged when using Q286R subcutaneous dosing Orphan Drug Designation Increased procoagulant activity Granted in the US and EU catalystbiosciences com 10

MarzAA Phase 2 demonstrates clinical efficacy Nasdaq: CBIO Greater than 90% reduction in all bleeding; Median ABR zero; Median bleeding days zero 19 8 12 3% Annualized Proportion of Days Bleed Rate with Bleeding n = 9 n = 9 1 6, p = 0 009 0 8%, p = 0 009 6 m pre-treatment On treatment 6 m pre-treatment On treatment Mean Annualized Bleeding Rates (ABR) significantly reduced from 19 8 to 1 6 Mean Proportion of Days with Bleeding (PDB) significantly reduced from 12 3% to 0 8% Safe & well tolerated, ~1% ISRs (6/517 SQ doses) and no ADAs catalystbiosciences com 11

In a world of SQ prophylaxis: Nasdaq: CBIO Patients need a SQ treatment of a bleed option Individuals on Hemlibra® SQ MarzAA meets the profile for an need additional treatments Ideal Solution  Fast & easy to administer NovoSeven® is safe but is administered IV Stops bleeding in a validated preclinical model Can be safely combined with FEIBA lacks a safety margin Hemlibra and is administered IV Blouse et al ASH 2019 catalystbiosciences com 12

MAA-102 PK dose levels supports treatment of a bleed Nasdaq: CBIO + Dose-proportional increases in Cmax and AUC + 25% of Cmax at 1 hour + 40% of Cmax at 1 5 hours + 50% of Cmax at 2 hours + No ADA Confidential 13

Marzeptacog alfa (activated) Nasdaq: CBIO Phase 3 studies to initiate in 2020 Large commercial opportunity across multiple rare bleeding disorders Demonstrated P2 Clinical efficacy & tolerability for prophylaxis indications Demonstrated preclinical PoC for SQ treatment of a bleed MarzAA can be safely combined with Hemlibra No SQ dose escalation PK/PD study supports treatment of a bleed – final data in Q2 2020 P3 guidance from EMA & MHRA received catalystbiosciences com 14

Dalcinonacog alfa: a novel SQ FIX product Nasdaq: CBIO Three substitutions within the FIX protein: + Increased catalytic activity + Higher affinity for FVIIIa + Resistance to antithrombin inhibition + 22-fold increased potency over BeneFIX Differentiated from marketed IV FIXs: + Simple SQ administration + Potential to maintain continuous protective levels + Small volume injection Resistance to + Enhanced pharmacokinetics with prolonged half-life antithrombin T343R R338E Increased FVIIIa affinity Orphan Drug Designation in US & EU & procoagulant activity catalystbiosciences com 15

Dalcinonacog alfa phase 2b SQ clinical trial design Nasdaq: CBIO Enrollment complete + Primary endpoint: Steady state FIX + 10 severe HB patients screened; 6 dosed activity level above 12% with daily dosing + Rare propeptide mutation excluded + Secondary endpoints: safety including weekly ADA testing, pharmacokinetics, pharmacodynamics, bleeding events, catalystbiosciences com 16

Target levels achieved with 100 IU/kg dosing for 28 Days Nasdaq: CBIO Target FIX >12% Achieved + Dosed 6 severe HB 110 continues 102 withdrew on Day 7 + Steady state FIX levels up to 27% achieved after 14 days + Consistent PK profiles + Terminal half-life is 70-112 hr + No breakthrough bleeds + No ADAs *Data cutoff 05 Feb 2020 catalystbiosciences com 17

Conclusions Nasdaq: CBIO + SQ dalcinonacog alfa provides stable therapeutic levels of Factor IX + Demonstrates the potential to be an effective prophylaxis treatment for individuals with Haemophilia B Trial enrollment complete Excellent & consistent therapeutic FIX activity levels attained Prolonged half-life with SQ administration No SAEs, systemic hypersensitivity, ADAs or nAb to DalcA or wild-type FIX Mild to moderate ISR’s primarily with initial injections No bleeding events during treatment demonstrates effective prophylaxis catalystbiosciences com 18

FIX gene therapy: CB 2679d-GT for hemophilia B Nasdaq: CBIO CB 2679d-GT in combination with a novel chimeric The 8x1010 vg/kg in hemophilia B mice AAV capsid provides significant improvements + Stable high activity levels in a mouse hemophilia B model – FIX AAV Study Dose FIX Activity Transgene Capsid (vg/kg) (U/mL) no nAbs Novel 10 CB 2679d-GT 8 0x10 20 + Vector dose reduced 10-fold compared to current constructs Chimeric + Potential for an improved efficacy & safety profile * 11 Padua TAK-748 7 4x10 20 + AAV license and sponsored research agreement with Padua TAK-748* 7 4x1010 1 Stanford University School of Medicine * Weiller et al (2019) Blood Vol 134, Supplement S1 P4633 Superior preclinical efficacy of CB 2679d-GT vs Padua + 4-5-fold reduction in bleeding time + Activity levels elevated throughout the study—no nAbs Wholly-owned & issued patents covering gene therapy catalystbiosciences com 19 Blouse et al EAHAD 2020

Targeting C3 blocks the downstream complement cascade Nasdaq: CBIO Lectin Pathway Classical Pathway Alternative Pathway C3 Convertase IVT CB 2782-PEG C3 Amplification Loop Antigen uptake Lysis C3a C3b Cell removal Cell death Secretion Inflammation C5 Convertase Proliferation C5a C5 C5b MAC Complex catalystbiosciences com 20

CB 2782-PEG anti-complement factor 3 (C3) protease Nasdaq: CBIO Geographic Atrophy in Dry AMD + Geographic atrophy is an advanced stage of dry age-related macular degeneration that results in the irreversible loss of retina and leads to blindness + Dry AMD affects a million people in the United States and over five million people worldwide + Global market is estimated at >$5B with no approved drugs + C3 is the only clinically validated target for the treatment of Dry AMD Sources: National Eye Institute Facts About Age-Related Macular Degeneration, Tufail 2015, The Eye Diseases Prevalence Research Group 2004, GlobalData catalystbiosciences com 21

CB 2782-PEG long acting anti-C3 protease Nasdaq: CBIO Best-in-class anti-C3 profile for dry AMD Biogen Collaboration + Generated from Catalyst’s proprietary + Announced December 19, 2019 protease engineering platform + $15M upfront, up to $ 340M in milestones + Potent, selective and long acting anti-C3 and tiered royalties up to low double digits protease that degrades C3 into inactive + Catalyst to perform fully funded pre-clinical fragments and manufacturing activities + Preclinical PK & PD data predict + Biogen responsible for IND-enabling best-in-class human intravitreal dosing activities, worldwide clinical development & three or four times a year commercialization + Dry AMD is a $5B+ market opportunity with no approved drugs catalystbiosciences com 22

Milestones Nasdaq: CBIO 2019 Q1 Q2 H2 MarzAA P2 efficacy EoP2 ToB enabling Registration (FVIIa) PK/PD Trial DalcA Positive P2b Update Final P2b data Interim data (FIX) CB 2679d-GT Preclinical efficacy NextGen Vector NHP Efficacy (FIX Gene Therapy) CB 2782-PEG Partnership (dAMD) catalystbiosciences com 23

Financial information Nasdaq: CBIO Selected data Financial results Q3 2019 Cash & Cash Equivalents    $85 0 M Operating Expense (YTD)    $43 3 M Net Loss (YTD)                ($41 6M) Net Loss per share (YTD)    ($3 47) Share data Common Stock Outstanding 12,029,992 Officer & Director ownership    7 0% Fully Diluted Shares*    14,859,051 * Includes ~1M options available for issuance catalystbiosciences com 24

Team Nasdaq: CBIO President & CEO SVP, Technical Operations Nassim Usman, Ph D Andrew Hetherington, M B A 26 years 20 years in biotech in biotech Chief Medical Officer VP, Translational Research Howard Levy, M B B Ch , Ph D , M M M Grant Blouse, Ph D 12 years 18 years in biotech in hematology VP, Business Development Jeffrey Landau, M B A 16 years in biotech catalystbiosciences 25

Summary Nasdaq: CBIO Disruptive approach to billion-dollar markets – protease engineering platform FVIIa: SQ MarzAA ~$2 2B market Anti-C3 dAMD: IVT CB 2782-PEG >$5B market + P2 efficacy & safety demonstrated + Biogen collaboration + P1/2 PK/PD supports ToB + $15M upfront, up to $340M in milestones and tiered royalties up to low double digits + FDA EoP2 in early 2020, P3 expected in 2020 FIX: SQ DalcA >$1 5B market SQ systemic complement inhibitor program + Phase 2b efficacy & safety demonstrated + Large orphan disease opportunity + Builds complement franchise + Final Phase 2b data in 2Q 2020 FIX Gene Therapy: CB 2679d-GT Strong financial position + Proprietary preclinical gene therapy asset with superior activity vs current clinical constructs catalystbiosciences com 26

THANK YOU Nasdaq: CBIO catalystbiosciences com

Exhibit 99.2

Catalyst Biosciences Presents Positive Data from its Phase 2b Trial of Subcutaneous Dalcinonacog Alfa

(DalcA) and Marzeptacog alfa (activated) (MarzAA) Programs at the 13^th Annual EAHAD Congress

Enrollment in the DalcA trial is complete; results demonstrate DalcA provides Factor IX (FIX) activity exceeding the efficacy endpoint with no anti-drug antibodies

Company to host investor call and webcast on Friday, February 7 at 8:30 a.m. ET

SOUTH SAN FRANCISCO, Calif., Feb. 7, 2020 — Catalyst Biosciences, Inc. (NASDAQ: CBIO), today announced positive efficacy and safety data from its Phase 2b trial of DalcA, a next-generation subcutaneously (SQ) administered Factor IX (FIX) therapy being developed for the treatment of hemophilia B. The data were presented by Johnny Mahlangu, M.B.B.Ch., M.Med, F.C.Path, professor of haematology, faculty of health sciences, head of the School of Pathology at the University of Witwatersrand in Johannesburg, South Africa, and principal investigator in the clinical trial in an oral presentation at the 13^th Annual Congress of the European Association for Haemophilia and Allied Disorders (EAHAD) in The Hague, Netherlands.

Data from the trial showed that 28 days of daily SQ dosing of DalcA achieved protective target FIX levels of >12%, with steady state FIX levels of up to 27% after 14 days with no bleeds, demonstrating effective prophylaxis and the potential for lower or less frequent dosing. One subject withdrew on day 7. No anti-drug antibodies were detected and no serious adverse events were reported. Three subjects reported injection site reactions (ISRs), the majority of which were mild in severity and resolved without sequelae.

The open-label Phase 2b study was designed to evaluate the ability of DalcA to maintain steady state protective FIX levels above 12% in six individuals with severe hemophilia B. Each subject received a single intravenous dose, followed by daily SQ doses of DalcA for 28 days. Pharmacokinetics, pharmacodynamics, safety and tolerability of daily SQ dosing and anti-drug antibody formation are being monitored.

“The DalcA Phase 2b trial data presented today at EAHAD clearly demonstrate the potential for DalcA to significantly change the treatment paradigm for those suffering from hemophilia B,” said Nassim Usman, Ph.D., president and chief executive officer of Catalyst. “With the trial now fully enrolled and one final subject completing dosing, we will announce final data in the second quarter of this year.”

Dr. Usman continued, “We are very encouraged by the data presented at EAHAD from three of our programs and see 2020 as a pivotal year for our entire hemophilia franchise. In addition to the Phase 2b DalcA data, our SQ FVIIa marzeptacog alfa (activated) MarzAA candidate has demonstrated efficacy and safety in individuals with hemophilia A or B with inhibitors in a Phase 2 prophylaxis study, and this week we presented pharmacokinetic and pharmacodynamic data that support SQ MarzAA use in acute or on-demand settings. Our high-potency FIX gene therapy candidate, CB 2679d-GT is developing into a promising asset with encouraging pre-clinical data using a proprietary next-generation AAV capsid. Our entire team is committed to developing novel treatments in multiple rare bleeding disorders.”

In addition to the oral presentation on the Phase 2b DalcA data, Catalyst presented three posters at the EAHAD congress. Dr. Linda Neuman, vice president, clinical development, presented data from a Phase 1 study to evaluate the pharmacokinetics, pharmacodynamics, and safety of ascending doses of SQ MarzAA in adult subjects with hemophilia, which showed that SQ dosing reaches target levels to treat ongoing bleeding. Dr. Grant Blouse, vice president, translational research, presented data on SQ MarzAA

demonstrating that on-demand treatment in Hemophilia A mice treated after a tail clip injury was as efficient as intravenous NovoSeven at reducing bleeding. Dr. Blouse also presented a poster on Hemophilia B gene therapy in mice demonstrating that a novel chimeric AAV capsid combined with the Company’s proprietary potency enhanced CB 2679d-GT FIX variant may reduce the vector dose required in gene therapy while maintaining high FIX levels.

A copy of the presentation materials can be accessed on the Events and Presentations section of the Catalyst website. Catalyst will host an investor call on Friday, February 7, at 8:30 a.m. ET.

Conference Call Details

The management team will host a conference call for investors on Friday, February 7 at 8:30 a.m. ET to discuss the DalcA and MarzAA data presented at EAHAD. Conference call, webcast and post-conference call replay details are as follows:

A webcast replay will be available for 30 days following the live event.

A copy of the presentation materials can be accessed on the Events and Presentations section of the Catalyst website.

About Catalyst Biosciences

Catalyst is a clinical-stage biopharmaceutical company focused on addressing unmet needs in rare diseases and systemic complement mediated disorders. Our protease engineering platform includes development programs in hemophilia and a research program on subcutaneous (SQ) systemic complement inhibitors. Our engineered coagulation factors are designed to overcome the significant limitations of current IV treatment options, facilitate prophylaxis, and ultimately deliver substantially better outcomes for patients using SQ dosing. Our lead asset, MarzAA has completed Phase 2 development having met its primary endpoint of significantly reducing the annualized bleed rate (ABR) in individuals with hemophilia A or B with inhibitors. Our second hemophilia asset, DalcA is in a Phase 2b clinical trial and is being developed for the treatment of hemophilia B. We also have a global license and collaboration agreement with Biogen for the development and commercialization of pegylated CB 2782 for the potential treatment of geographic atrophy associated dry age-related macular degeneration. For more information, please visit www.catalystbiosciences.com.

Forward-Looking Statements

This press release contains forward-looking statements that involve substantial risks and uncertainties. Forward-looking statements include statements about the potential uses and benefits of DalcA to provide benefits and change the treatment paradigm for patients with hemophilia B and for MarzAA to treat patients with hemophilia A or B with inhibitors, the potential benefits of SQ dosing, statements about Catalyst’s clinical trial status for DalcA, the potential for CB 2679d-GT to be a promising asset and the potential use of MarzAA as an on-demand therapy. Actual results or events could differ materially from the plans, intentions, expectations and projections disclosed in the forward-looking statements. Various important factors could cause actual results or events to differ materially, including, but not limited to, the risk that trials and studies may be delayed and may not have satisfactory outcomes, that additional

human trials will not replicate the results from earlier trials, that potential adverse effects may arise from the testing or use of DalcA, or MarzAA, including the generation of antibodies, which has been observed in patients previously treated with DalcA, the risk that costs required to develop or manufacture the Company’s products will be higher than anticipated, competition and other risks described in the “Risk Factors” section of the Company’s quarterly report filed with the Securities and Exchange Commission on November 7, 2019, and in other filings with the Securities and Exchange Commission. The Company does not assume any obligation to update any forward-looking statements, except as required by law.

Contact:

Ana Kapor

Catalyst Biosciences, Inc.

investors@catbio.com