Equity Transactions	6 Months Ended
Jun. 30, 2021
Equity [Abstract]
Equity Transactions	5. Equity Transactions Underwritten Public Offering On September 15, 2020, the Company completed an underwritten public offering of an aggregate of 4,785,000 shares of its common stock, and an aggregate of 3,215,000 shares of newly designated Series D Convertible Preferred Stock (“Series D Preferred Stock”). The price to the public in the offering was $10.00 per share with respect to the common stock and the Series D Preferred Stock. In addition, under the terms of the Underwriting Agreement, the Company granted the underwriters an option, exercisable for 30 days, to purchase up to 1,200,000 additional shares of common stock. The shares of Series D Preferred Stock are convertible on a one-to-one basis into shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series D Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99% of the total number of shares of common stock then issued and outstanding, subject to certain exceptions. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series D Preferred Stock will receive a payment equal to $0.001 per share of Series D Preferred Stock before any proceeds are distributed to the holders of common stock and equal to any distributions to the holders of Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock. Shares of Series D Preferred Stock will generally have no voting rights, except as required by law and except that the consent of holders of a majority of the then outstanding Series D Preferred Stock will be required to amend the terms of the Series D Preferred Stock. As such, the Company has classified the Series D Preferred Stock within permanent equity in its consolidated balance sheet. The offering closed on September 15, 2020 with an aggregate public offering price of $80.0 million. Aggregate net proceeds from the offering were $74.7 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. Additionally, pursuant to the Underwriting Agreement, on October 1, 2020, the Company issued and sold 1,200,000 shares of common stock at the price of $10.00 per share pursuant to the underwriters’ exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of approximately $11.2 million after deducting underwriting discounts and commissions. Rights Offering On February 11, 2020, the Company announced a rights offering pursuant to which it distributed to holders of its common stock and Series A Convertible Preferred Stock (“Series A Preferred Stock”) and Series B Convertible Preferred Stock (“Series B Preferred Stock”), at no charge, non-transferable subscription rights to purchase shares of Spero common stock and Series C Convertible Preferred Stock (“Series C Preferred Stock”), with an aggregate offering value of $30.0 million. For each share of common stock (including shares of common stock issuable upon conversion of the Company’s outstanding shares of Series A Preferred Stock and Series B Preferred Stock) owned by holders of record as of 5:00 p.m., New York time, on February 10, 2020, the holders of such shares received 0.152 rights to purchase shares of Spero common stock (subject to the aggregate offering threshold and certain ownership limitations). Each whole right allowed holders to subscribe for one share of common stock at the subscription price equal to $9.00 per whole share (or an equivalent number of shares of Series C Preferred Stock). The total number of subscription rights issued to each stockholder was rounded down to the nearest whole number. The Rights Offering was fully backstopped by certain affiliates of BVF Partners L.P. (“BVF”), which agreed to purchase, at a minimum, their respective as-converted pro rata share of the offered shares under the Rights Offering, plus an additional amount of Common Stock or Series C Preferred Stock that are not subscribed by other purchasers in the Rights Offering, for a total of up to $30.0 million. At the closing of the rights offering on March 5, 2020, a total of 1,046,249 shares of the Company’s common stock and 2,287 shares of Series C Preferred Stock were issued for aggregate gross proceeds of $30.0 million. The aggregate issue costs related to the offering were $0.5 million. $20.6 million of the aggregate gross proceeds relates to the issuance of Series C and the associated issuance costs are $0.1 million. Each share of Series C Preferred Stock is convertible into 1,000 shares of Spero common stock at the election of the holder, subject to beneficial ownership conversion limits applicable to the Series C Preferred Stock. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series C Preferred Stock will receive a payment equal to $0.001 per share of Series C Preferred Stock before any proceeds are distributed to the holders of Common Stock and equal to any distributions to the holders of the Series A Preferred Stock and Series B Preferred Stock. The Series C Preferred Stock have no voting rights, except as required by law. The Series C Preferred Stock does not have any mandatory redemption rights or other redemption rights that would be outside of the Company’s control. As such, the Company has classified the Series C Preferred Stock within permanent equity in its consolidated balance sheet. Upon issuance, each share of Series C Preferred Stock included an embedded beneficial conversion feature. The beneficial conversion feature arose because the market price of the Company’s common stock on the date of issuance of the Series C Preferred Stock was $9.22 per share as compared to an effective conversion price of the Series C Preferred Stock of $8.98 per share. As a result, the Company recorded the intrinsic value of the beneficial conversion feature of $0.5 million as a discount on the Series C Preferred Stock at issuance. Because the Series C Preferred Stock is immediately convertible upon issuance and does not include mandatory redemption provisions, the discount on the Series C Preferred Stock was immediately accreted.

Equity Transactions

6 Months Ended

Jun. 30, 2021

5. Equity Transactions

Underwritten Public Offering

On September 15, 2020, the Company completed an underwritten public offering of an aggregate of 4,785,000 shares of its common stock, and an aggregate of 3,215,000 shares of newly designated Series D Convertible Preferred Stock (“Series D Preferred Stock”). The price to the public in the offering was $10.00 per share with respect to the common stock and the Series D Preferred Stock. In addition, under the terms of the Underwriting Agreement, the Company granted the underwriters an option, exercisable for 30 days, to purchase up to 1,200,000 additional shares of common stock.

The shares of Series D Preferred Stock are convertible on a one-to-one basis into shares of common stock at any time at the option of the holder, provided that the holder will be prohibited from converting the Series D Preferred Stock into shares of common stock if, as a result of such conversion, the holder, together with its affiliates, would own more than 9.99% of the total number of shares of common stock then issued and outstanding, subject to certain exceptions. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series D Preferred Stock will receive a payment equal to $0.001 per share of Series D Preferred Stock before any proceeds are distributed to the holders of common stock and equal to any distributions to the holders of Series A Preferred Stock, Series B Preferred Stock and Series C Preferred Stock. Shares of Series D Preferred Stock will generally have no voting rights, except as required by law and except that the consent of holders of a majority of the then outstanding Series D Preferred Stock will be required to amend the terms of the Series D Preferred Stock. As such, the Company has classified the Series D Preferred Stock within permanent equity in its consolidated balance sheet.

The offering closed on September 15, 2020 with an aggregate public offering price of $80.0 million. Aggregate net proceeds from the offering were $74.7 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by the Company. Additionally, pursuant to the Underwriting Agreement, on October 1, 2020, the Company issued and sold 1,200,000 shares of common stock at the price of $10.00 per share pursuant to the underwriters’ exercise of their option to purchase additional shares of common stock, resulting in additional net proceeds of approximately $11.2 million after deducting underwriting discounts and commissions.

Rights Offering

On February 11, 2020, the Company announced a rights offering pursuant to which it distributed to holders of its common stock and Series A Convertible Preferred Stock (“Series A Preferred Stock”) and Series B Convertible Preferred Stock (“Series B Preferred Stock”), at no charge, non-transferable subscription rights to purchase shares of Spero common stock and Series C Convertible Preferred Stock (“Series C Preferred Stock”), with an aggregate offering value of $30.0 million. For each share of common stock (including shares of common stock issuable upon conversion of the Company’s outstanding shares of Series A Preferred Stock and Series B Preferred Stock) owned by holders of record as of 5:00 p.m., New York time, on February 10, 2020, the holders of such shares received 0.152 rights to purchase shares of Spero common stock (subject to the aggregate offering threshold and certain ownership limitations). Each whole right allowed holders to subscribe for one share of common stock at the subscription price equal to $9.00 per whole share (or an equivalent number of shares of Series C Preferred Stock). The total number of subscription rights issued to each stockholder was rounded down to the nearest whole number.

The Rights Offering was fully backstopped by certain affiliates of BVF Partners L.P. (“BVF”), which agreed to purchase, at a minimum, their respective as-converted pro rata share of the offered shares under the Rights Offering, plus an additional amount of Common Stock or Series C Preferred Stock that are not subscribed by other purchasers in the Rights Offering, for a total of up to $30.0 million.

At the closing of the rights offering on March 5, 2020, a total of 1,046,249 shares of the Company’s common stock and 2,287 shares of Series C Preferred Stock were issued for aggregate gross proceeds of $30.0 million. The aggregate issue costs related

to the offering were $0.5 million. $20.6 million of the aggregate gross proceeds relates to the issuance of Series C and the associated issuance costs are $0.1 million.

Each share of Series C Preferred Stock is convertible into 1,000 shares of Spero common stock at the election of the holder, subject to beneficial ownership conversion limits applicable to the Series C Preferred Stock. In the event of the Company’s liquidation, dissolution, or winding up, holders of Series C Preferred Stock will receive a payment equal to $0.001 per share of Series C Preferred Stock before any proceeds are distributed to the holders of Common Stock and equal to any distributions to the holders of the Series A Preferred Stock and Series B Preferred Stock. The Series C Preferred Stock have no voting rights, except as required by law. The Series C Preferred Stock does not have any mandatory redemption rights or other redemption rights that would be outside of the Company’s control. As such, the Company has classified the Series C Preferred Stock within permanent equity in its consolidated balance sheet.

Upon issuance, each share of Series C Preferred Stock included an embedded beneficial conversion feature. The beneficial conversion feature arose because the market price of the Company’s common stock on the date of issuance of the Series C Preferred Stock was $9.22 per share as compared to an effective conversion price of the Series C Preferred Stock of $8.98 per share. As a result, the Company recorded the intrinsic value of the beneficial conversion feature of $0.5 million as a discount on the Series C Preferred Stock at issuance. Because the Series C Preferred Stock is immediately convertible upon issuance and does not include mandatory redemption provisions, the discount on the Series C Preferred Stock was immediately accreted.