X0206 3 2017-11-15 0 0001501697 Arsanis, Inc. ASNS 0001055951 ORBIMED ADVISORS LLC 601 LEXINGTON AVE., 54TH FLOOR NEW YORK NY 10022 0 0 1 0 0001502240 OrbiMed Capital GP IV LLC 601 LEXINGTON AVE, 54TH FLOOR NEW YORK NY 10022 0 0 1 0 0001055949 ISALY SAMUEL D 601 LEXINGTON AVE, 54TH FLOOR NEW YORK NY 10022 0 0 1 0 Series A-1 Convertible Preferred Stock Common Stock 19533 I See Footnotes Series A-2 Convertible Preferred Stock Common Stock 252230 I See Footnotes Series B Convertible Preferred Stock Common Stock 328909 I See Footnotes Series C Convertible Preferred Stock Common Stock 114320 I See Footnotes Series D Convertible Preferred Stock Common Stock 653974 I See Footnotes The Series A-1 Convertible Preferred Stock is convertible into Common Stock on a 0.29300-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. The Series A-2 Convertible Preferred Stock is convertible into Common Stock on a 0.35785-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. The Series B Convertible Preferred Stock is convertible into Common Stock on a 0.44650-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. The Series C Convertible Preferred Stock is convertible into Common Stock on a 0.49832-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. The Series D Convertible Preferred Stock is convertible into Common Stock on a 0.29300-for-one basis into the number of shares of Common Stock shown in column 3 at any time at the holder's election and automatically upon the closing of the Issuer's initial public offering without payment of further consideration. The shares have no expiration date. The reported securities are held of record by OrbiMed Private Investments IV, LP ("OPI IV"). OrbiMed Capital GP IV LLC ("GP IV") is the sole general partner of OPI IV, and OrbiMed Advisors LLC ("Advisors"), a registered adviser under the Investment Advisers Act of 1940, as amended, is the sole managing member of GP IV. Samuel D. Isaly ("Isaly"), a natural person, is the managing member of, and owner of a controlling interest in, Advisors. By virtue of such relationships, GP IV, Advisors and Isaly may be deemed to have voting and investment power with respect to the securities held by OPI IV. The Reporting Persons have designated a representative, currently Carl Gordon, a member of Advisors, to serve on the Issuer's board of directors. This report on Form 3 is jointly filed by GP IV, Advisors and Isaly. Each of the reporting persons disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its or his pecuniary interest therein, if any. This report on Form 3 shall not be deemed an admission that any of the reporting persons is a beneficial owner of such securities for the purpose of Section 16 of the Exchange Act, or for any other purpose. /s/ Samuel D. Isaly, as managing member of OrbiMed Advisors LLC 2017-11-15 /s/ Samuel D. Isaly, as managing member of OrbiMed Advisors LLC, the managing member of OrbiMed Capital GP IV LLC 2017-11-15 /s/ Samuel D. Isaly 2017-11-15