1018-00001 · Client 1018 · Opinions & Comfort Letters

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CALDERWOOD & HARKNESS LLP Attorneys at Law

One Liberty Plaza, 52nd Floor New York, New York 10006

Telephone: (212) 558-4000 Facsimile: (212) 558-3588 www.calderwoodharkness.com

October 17, 2023

Ridgeline Securities LLC, as Representative of the Several Underwriters 383 Madison Avenue New York, New York 10017

Broadmark Capital Inc. 200 Park Avenue New York, New York 10166

Linden Groves & Co. 745 Fifth Avenue New York, New York 10151

Harborview Partners LLC 1 Federal Street Boston, Massachusetts 02110

(each of the foregoing, collectively, the "Underwriters")

Ladies and Gentlemen:

We have acted as counsel to Penterra Pharmaceuticals Corp., a Delaware corporation (the "Company" or "PPC"), in connection with the public offering by the selling stockholders named herein (the "Selling Stockholders") of shares of common stock, par value $0.001 per share (the "Common Stock"), of the Company pursuant to the Underwriting Agreement dated October 12, 2023 (the "Underwriting Agreement") among the Company, the Selling Stockholders named therein, and the several Underwriters named in Schedule I thereto, for whom Ridgeline Securities LLC and Broadmark Capital Inc. are acting as Representatives (the "Representatives"). Capitalized terms used but not otherwise defined herein have the meanings assigned to them in the Underwriting Agreement.

Pursuant to the Underwriting Agreement, the Selling Stockholders have agreed to sell to the several Underwriters an aggregate of 6,250,000 shares of Common Stock (the "Shares"). In addition, the Selling Stockholders have granted to the Underwriters an option, exercisable for a period of thirty (30) days from the date of the Underwriting Agreement, to purchase up to an additional 937,500 shares of Common Stock (the "Option Shares" and, together with the Shares, the "Securities") solely to cover over-allotments, if any.

This opinion letter is being furnished to you pursuant to Section 7(a) of the Underwriting Agreement as a condition to the obligations of the several Underwriters to purchase the Shares at the closing scheduled to occur on the date hereof (the "Closing Date"). This opinion letter is being delivered to you simultaneously with the closing of the sale of the Shares by the Selling Stockholders to the Underwriters.

I. IDENTIFICATION OF SELLING STOCKHOLDERS

The Selling Stockholders, and the number of shares of Common Stock being sold by each in the base offering, are as follows:

The Option Shares, if any, will be sold by the Selling Stockholders in proportions to be determined in accordance with the terms of the Underwriting Agreement upon the exercise of the Underwriters' over-allotment option.

II. DOCUMENTS EXAMINED

In connection with the opinions expressed herein, we have examined originals or copies, certified or otherwise identified to our satisfaction, of the following documents:

1. The Restated Certificate of Incorporation of the Company, as amended through the date hereof, as filed with the Secretary of State of the State of Delaware (the "Certificate of Incorporation");

2. The Amended and Restated Bylaws of the Company, as currently in effect (the "Bylaws");

3. Resolutions of the Board of Directors of the Company duly adopted at a meeting held on September 8, 2023, relating to the offering of the Securities and the Company's cooperation therewith, including the authorization of the execution and delivery of the Underwriting Agreement and related documents (the "Board Resolutions");

4. The Underwriting Agreement dated October 12, 2023, among the Company, the Selling Stockholders, and the several Underwriters;

5. The Company's Registration Statement on Form S-3 (File No. 333-274851), as filed with the Securities and Exchange Commission (the "SEC") under the Securities Act of 1933, as amended (the "Securities Act"), and declared effective by the SEC on August 22, 2023 (the "Registration Statement"), including the base prospectus contained therein (the "Base Prospectus"), together with all exhibits thereto and documents incorporated by reference therein;

6. The Preliminary Prospectus Supplement dated October 2, 2023, relating to the offering of the Securities, as filed with the SEC (the "Preliminary Prospectus Supplement");

7. The Final Prospectus Supplement dated October 12, 2023, relating to the offering of the Securities, as filed with the SEC pursuant to Rule 424(b)(5) under the Securities Act (the "Prospectus Supplement" and, together with the Base Prospectus, the "Prospectus");

8. The Issuer Free Writing Prospectus in the form of a Pricing Term Sheet dated October 12, 2023, as filed with the SEC pursuant to Rule 433 under the Securities Act (the "Pricing Term Sheet");

9. A certificate of good standing with respect to Penterra Pharmaceuticals Corp. issued by the Secretary of State of the State of Delaware, dated October 16, 2023 (the "Good Standing Certificate");

10. An Officer's Certificate of the Company dated October 17, 2023, executed by Dr. Marco Castellano, Chief Executive Officer of the Company, and Lisa Hargrove, Chief Financial Officer of the Company (the "Officer's Certificate");

11. A Secretary's Certificate of the Company dated October 17, 2023, executed by David Orenstein, General Counsel and Corporate Secretary of the Company, certifying as to the incumbency of officers, the Certificate of Incorporation, the Bylaws, and the Board Resolutions (the "Secretary's Certificate");

12. Stock records and transfer agent records of Pinnacle Trust & Transfer Co., in its capacity as the transfer agent and registrar for the Common Stock of the Company, with respect to the Shares and the Option Shares; and

13. Such other documents, records, certificates, corporate proceedings, and instruments as we have deemed necessary or appropriate as a basis for the opinions expressed herein.

The documents described in items (1) through (4) above are hereinafter referred to collectively as the "Transaction Documents." The Registration Statement, the Base Prospectus, the Preliminary Prospectus Supplement, the Prospectus Supplement, and the Pricing Term Sheet are hereinafter referred to collectively as the "Offering Documents."

III. ASSUMPTIONS

In rendering the opinions set forth herein, we have, with your consent and without independent investigation or verification, assumed the following:

(a) The genuineness of all signatures on all documents examined by us (other than signatures of persons signing on behalf of the Company);

(b) The authenticity and completeness of all documents submitted to us as originals;

(c) The conformity to the originals of all documents submitted to us as copies, whether certified, conformed, photostatic, electronic, or otherwise, and the authenticity and completeness of such originals;

(d) The legal capacity of all natural persons who have executed documents reviewed by us;

(e) Each party to the Underwriting Agreement (other than the Company) has the requisite corporate or other organizational power, authority, and legal right to execute, deliver, and perform its respective obligations under the Underwriting Agreement, and the Underwriting Agreement has been duly authorized, executed, and delivered by each such party;

(f) The Underwriting Agreement constitutes the legal, valid, and binding obligation of each party thereto (other than the Company), enforceable against each such party in accordance with its terms;

(g) The accuracy and completeness of all representations and warranties made to us by officers and other representatives of the Company in the Officer's Certificate, the Secretary's Certificate, and in other certificates and communications, whether written or oral;

(h) The Shares and the Option Shares, as applicable, have been or will be delivered through the book-entry facilities of The Depository Trust Company ("DTC") in accordance with the terms of the Underwriting Agreement and the customary settlement procedures of DTC;

(i) The Board Resolutions certified in the Secretary's Certificate have not been amended, modified, rescinded, or revoked and remain in full force and effect as of the date hereof; and

(j) There are no agreements, understandings, or arrangements, whether written or oral, between or among the parties to the Underwriting Agreement that would modify, supplement, or otherwise affect any of the Transaction Documents or the transactions contemplated thereby, other than the Transaction Documents themselves and the documents delivered at the closing of the offering on the date hereof.

IV. OPINIONS

Based upon and subject to the foregoing, and subject to the qualifications and limitations set forth in Section V below, we are of the opinion that:

Opinion 1 — Due Incorporation and Good Standing

The Company has been duly incorporated and is validly existing as a corporation in good standing under the laws of the State of Delaware. In rendering this opinion, we have relied exclusively upon the Good Standing Certificate issued by the Secretary of State of the State of Delaware dated October 16, 2023, and our review of the Certificate of Incorporation and the Secretary's Certificate. We note that the Good Standing Certificate confirms that the Company was incorporated on March 14, 2009, and that it is in good standing as of the date of such certificate, having filed all required annual reports and paid all franchise taxes due and owing to the State of Delaware through the date of such certificate.

Opinion 2 — Authorized Capital Stock

The authorized capital stock of the Company consists of (i) 500,000,000 shares of Common Stock, par value $0.001 per share, and (ii) 25,000,000 shares of preferred stock, par value $0.001 per share, in each case as set forth in the Certificate of Incorporation and as described in the Prospectus under the caption "Description of Capital Stock." Based upon our review of the stock records and transfer agent records of Pinnacle Trust & Transfer Co. and the Officer's Certificate, the Company has 120,000,000 shares of Common Stock issued and outstanding as of the date hereof, and no shares of preferred stock issued and outstanding as of the date hereof. All such outstanding shares have been duly authorized, validly issued, and are fully paid and non-assessable.

Opinion 3 — Valid Issuance of the Shares

The 6,250,000 shares of Common Stock being sold by the Selling Stockholders in the base offering and the up to 937,500 Option Shares that may be sold by the Selling Stockholders upon exercise of the Underwriters' over-allotment option have been duly authorized, are validly issued, fully paid, and non-assessable. The issuance of such shares of Common Stock was not and is not subject to any preemptive rights or similar rights under the Certificate of Incorporation or the Bylaws or, to our knowledge, any agreement or instrument to which the Company is a party or by which the Company or any of its properties is bound. The holders of such shares are not and will not be subject to personal liability solely by reason of being such holders. The shares of Common Stock comprising the Securities conform in all material respects to the description of the Common Stock contained in the Prospectus under the caption "Description of Capital Stock."

Opinion 4 — Registration Statement Effectiveness

The Registration Statement on Form S-3 (File No. 333-274851) has been filed with the SEC and declared effective under the Securities Act as of August 22, 2023. To our knowledge, no stop order suspending the effectiveness of the Registration Statement has been issued by the SEC, and no proceedings for that purpose have been instituted, are pending, or have been threatened by the SEC as of the date hereof. The Prospectus Supplement has been filed with the SEC in accordance with Rule 424(b)(5) under the Securities Act. The Pricing Term Sheet has been filed with the SEC in accordance with Rule 433 under the Securities Act. To our knowledge, the Company has complied with the conditions for use of Form S-3 under the Securities Act in connection with the offering of the Securities.

Opinion 5 — Authorization of the Underwriting Agreement

The Underwriting Agreement has been duly authorized by all necessary corporate action on the part of the Company, including the adoption of the Board Resolutions at a duly convened meeting of the Board of Directors of the Company held on September 8, 2023 at which a quorum was present and acting throughout. The Underwriting Agreement has been duly executed and delivered by the Company. The Company has full corporate power and authority under the General Corporation Law of the State of Delaware, the Certificate of Incorporation, and the Bylaws to execute, deliver, and perform its obligations under the Underwriting Agreement and to consummate the transactions contemplated thereby.

Opinion 6 — No Conflicts

The execution, delivery, and performance by the Company of its obligations under the Underwriting Agreement, and the consummation of the transactions contemplated thereby (including the cooperation of the Company in the sale of the Securities by the Selling Stockholders), do not and will not:

(a) violate, conflict with, or result in a breach of any provision of the Restated Certificate of Incorporation or the Amended and Restated Bylaws of the Company;

(b) to our knowledge, violate any existing applicable statute, law, rule, regulation, judgment, order, writ, injunction, or decree of any court or governmental agency or body of the State of New York, the State of Delaware, or the federal laws of the United States of America that is applicable to the Company or any of its properties or assets; or

(c) to our knowledge, result in a breach of, constitute a default (or an event that, with or without notice or lapse of time or both, would constitute a default) under, give rise to any right of termination, cancellation, acceleration, or loss of any benefit under, or result in the creation or imposition of any lien, charge, security interest, or encumbrance upon any of the properties or assets of the Company pursuant to, any material agreement, indenture, mortgage, deed of trust, loan agreement, lease, license, or other instrument to which the Company is a party or by which the Company or any of its properties or assets is bound.

Opinion 7 — No Consents Required

No consent, approval, authorization, order, filing, registration, or qualification of or with any federal, state, or local court, governmental agency, or regulatory body is required for the execution, delivery, and performance by the Company of its obligations under the Underwriting Agreement, or the consummation by the Company of the transactions contemplated thereby, except for (i) such as have already been obtained or made, including the effectiveness of the Registration Statement under the Securities Act and the filing of the Prospectus Supplement with the SEC pursuant to Rule 424(b)(5) under the Securities Act, (ii) such as may be required under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), in connection with the Company's reporting obligations, and (iii) such as may be required under state securities or "blue sky" laws in connection with the purchase and distribution of the Securities by the Underwriters, as to which state securities or "blue sky" laws we express no opinion.

Opinion 8 — Nasdaq Listing

The shares of Common Stock comprising the Securities have been approved for listing on the Nasdaq Global Select Market under the ticker symbol "PNTR," subject only to official notice of issuance. The Common Stock is registered under Section 12(b) of the Exchange Act. The CUSIP number assigned to the Common Stock is 70831X 10 5.

V. QUALIFICATIONS AND LIMITATIONS

The foregoing opinions are subject to the following qualifications and limitations:

(a) Governing Law Limitation. Our opinions are limited solely to the laws of the State of New York, the General Corporation Law of the State of Delaware (including the statutory provisions thereof, all applicable provisions of the Delaware Constitution, and reported judicial decisions interpreting the foregoing), and the federal laws of the United States of America, each as in effect on the date hereof. We express no opinion as to the laws of any other jurisdiction, and we assume no obligation to advise you of any change in applicable law after the date hereof or of any facts or circumstances that may come to our attention after the date hereof that might alter the opinions expressed herein.

(b) State Securities Laws. We express no opinion with respect to state securities or "blue sky" laws of any state or other jurisdiction, except to the extent expressly stated in Opinion 4 above with respect to federal securities laws.

(c) Good Standing Reliance. Our opinion in Opinion 1 regarding the good standing of the Company under the laws of the State of Delaware is based solely upon, and is limited by, the Good Standing Certificate issued by the Secretary of State of the State of Delaware dated October 16, 2023. We have made no independent investigation of the Company's compliance with Delaware franchise tax obligations or annual reporting requirements beyond our review of such certificate.

(d) Date of Opinion; No Obligation to Update. The opinions expressed herein are rendered as of the date hereof, and we assume no obligation to update or supplement this opinion letter to reflect any facts or circumstances that may hereafter come to our attention, or any changes in applicable law that may hereafter occur, whether or not such facts, circumstances, or changes might affect the opinions expressed herein. Without limiting the foregoing, we assume no obligation to update this opinion letter upon the exercise of the Underwriters' over-allotment option with respect to the Option Shares.

(e) Purpose Limitation. This opinion letter is rendered solely in connection with the transactions contemplated by the Underwriting Agreement, including the sale of the Securities by the Selling Stockholders to the Underwriters and the related offering of the Securities to the public, and may not be relied upon by any person or entity for any other purpose.

(f) Factual Reliance. As to all matters of fact (as distinguished from matters of law), we have relied upon certificates, representations, and statements of public officials and officers and representatives of the Company, including the Officer's Certificate and the Secretary's Certificate, and have not independently verified the accuracy or completeness of such certificates, representations, or statements. Where our opinions are qualified by the phrase "to our knowledge," such phrase means the actual, conscious knowledge of Jennifer Kwon, Theodore Langston, Audrey Zhou, and Brendan Walsh, the attorneys at this firm who have given substantive legal attention to the representation of the Company in connection with this offering, as of the date hereof, without having undertaken any independent investigation or inquiry for purposes of this opinion letter. We have not conducted any search of any docket, index, or filing system of any court, governmental body, or regulatory authority, or reviewed any database or other publicly available source of information, for purposes of rendering this opinion, except as expressly described herein.

(g) Enforceability Qualifications. To the extent any opinion set forth herein relates to the enforceability of any agreement or obligation, such opinion is subject to (i) applicable bankruptcy, insolvency, reorganization, moratorium, receivership, conservatorship, liquidation, fraudulent conveyance, fraudulent transfer, preferential transfer, and similar laws affecting the rights and remedies of creditors generally, (ii) general principles of equity (whether considered in a proceeding at law or in equity), including concepts of materiality, reasonableness, good faith, and fair dealing, and the possible unavailability of specific performance, injunctive relief, or other equitable remedies regardless of whether enforceability is considered in a proceeding in equity or at law, and (iii) the discretion of any court of competent jurisdiction in awarding attorneys' fees, costs, or other relief.

VI. RELIANCE AND BENEFIT

This opinion letter is furnished to the Underwriters solely for their benefit in connection with the offering of the Securities and may not be relied upon by any other person or entity without our prior written consent. Without limiting the foregoing, this opinion letter may not be quoted in whole or in part, referred to in any report or document, or furnished to any other person or entity, without our prior written consent in each instance, except that:

(a) this opinion letter may be filed as an exhibit to the Registration Statement, to any Current Report on Form 8-K filed by the Company in connection with the offering, or to any other report or filing made by the Company with the SEC, in each case as required by applicable law or regulation;

(b) this opinion letter may be disclosed to the extent required by applicable law, regulation, or judicial or administrative process, provided that the disclosing party shall use reasonable efforts to provide us with prompt prior written notice of any such required disclosure (to the extent legally permissible); and

(c) this opinion letter may be relied upon by any person or entity who becomes an Underwriter pursuant to the terms of the Underwriting Agreement.

We consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the reference to our firm under the caption "Legal Matters" in the Prospectus. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the SEC thereunder.

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Very truly yours,