Form 4: Mirum Director Sells Shares After Option Exercise

Mirum Pharmaceuticals director Michael G. Grey exercised stock options and subsequently sold 50,000 shares of common stock for $74 per share.

Summary

• Michael G. Grey, a director of Mirum Pharmaceuticals, Inc. (MIRM), engaged in a transaction on December 1, 2025.
• He exercised a stock option to acquire 50,000 shares of common stock at an exercise price of $2.936 per share.
• Immediately following the option exercise, he sold all 50,000 newly acquired shares at a price of $74 per share.
• This sale was conducted under a pre-arranged Rule 10b5-1 trading plan, which was adopted on August 22, 2025.
• After these transactions, Mr. Grey directly owns 0 shares of common stock.
• He continues to indirectly own 186,410 shares through The Grey Family Trust and 93,750 shares through The Grey 2018 Irrevocable Children's Trust, totaling 280,160 indirect shares.
• His remaining beneficial ownership of derivative securities (stock options) is 231,176.

Sentiment

Score: 6

Explanation: The transaction represents a director monetizing vested options at a significantly higher price than the exercise price, indicating strong stock performance. The use of a 10b5-1 plan suggests a pre-planned financial event rather than a reaction to new information. While insider selling can sometimes be viewed negatively, the context of option exercise and a pre-arranged plan makes it less concerning.

Positives

• The director realized a significant gain from exercising options at $2.936 and selling at $74, indicating a substantial increase in the company's stock value since the options were granted.
• The sale was conducted under a Rule 10b5-1 plan, which suggests a pre-planned transaction rather than a reaction to immediate news, potentially mitigating concerns about opportunistic insider selling.

Negatives

• A director selling a significant number of shares, even under a 10b5-1 plan, could be perceived negatively by some investors as it reduces their direct stake in the company.

Future Outlook

NA

Industry Context

This is an insider transaction report, reflecting a director's personal financial planning rather than a direct commentary on broader industry trends or competitive landscape. However, the high sale price indicates strong past performance of the company's stock within its industry.

Related Party Transactions

• Michael G. Grey, a director of Mirum Pharmaceuticals, Inc., engaged in the exercise of stock options and subsequent sale of common stock. This is a standard insider transaction.

Stakeholder Impact

• Shareholders: May view the director's sale as a signal, though the 10b5-1 plan mitigates immediate concerns. The significant profit realized by the director highlights the stock's appreciation, which is positive for long-term shareholders.

Key Dates

Recommendation

The director's sale, while significant in volume, was executed under a pre-arranged 10b5-1 plan, which typically signals a personal financial planning event rather than a bearish view on the company's immediate prospects. The substantial profit realized from the option exercise and sale at $74 per share reflects strong past performance of Mirum Pharmaceuticals' stock. Given the pre-planned nature and the director's continued indirect ownership, this transaction alone does not warrant a change in investment thesis. Investors should hold and monitor future company performance and broader market conditions.

Keywords

Mirum Pharmaceuticals, MIRM, SEC Form 4, Insider Trading, Stock Option Exercise, Share Sale, Director Transaction, Rule 10b5-1 Plan, Beneficial Ownership